diff --git a/.github/workflows/workflow.yml b/.github/workflows/workflow.yml
index 995e469628..ee4132a53b 100644
--- a/.github/workflows/workflow.yml
+++ b/.github/workflows/workflow.yml
@@ -25,7 +25,7 @@ jobs:
         uses: actions/checkout@v2
 
       - name: Use OCaml ${{ matrix.ocaml-version }}
-        uses: actions-ml/setup-ocaml@master
+        uses: ocaml/setup-ocaml@v2
         with:
           ocaml-version: ${{ matrix.ocaml-version }}
           opam-depext: false
diff --git a/CHANGES.md b/CHANGES.md
index 277ec5eb88..68eee4a01a 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -8,11 +8,14 @@
   stack overflow happens in the XHR completion handler (mefyl #762).
 - lwt_jsoo: Add test suite (mefyl #764).
 
-- Cohttp.Header: new implementation (@lyrm #747)
+- Cohttp.Header: new implementation (lyrm #747)
 
   + New implementation of Header modules using an associative list instead of a map, with one major semantic change (function ```get```, see below), and some new functions (```clean_dup```, ```get_multi_concat```)
   + More Alcotest tests as well as fuzzing tests for this particular module.
 
+- Cohttp.Header: performance improvement (mseri, anuragsoni #778)
+  **Breaking** the headers are no-longer lowercased when parsed, the headers key comparison is case insensitive instead.
+
   ### Purpose
 
   The new header implementation uses an associative list instead of a map to represent headers and is focused on predictability and intuitivity: except for some specific and documented functions, the headers are always kept in transmission order, which makes debugging easier and is also important for [RFC7230§3.2.2](https://tools.ietf.org/html/rfc7230#section-3.2.2) that states that multiple values of a header must be kept in order.
@@ -38,6 +41,7 @@
 
   + ```clean_dup```  enables the user to clean headers that follows the {{:https://tools.ietf.org/html/rfc7230#section-3.2.2} RFC7230§3.2.2} (no duplicate, except ```set-cookie```)
   + ```get_multi_concat``` has been added to get a result similar to the previous ```get``` function.
+- Cohttp.Header: optimize internal of cohttp.headers (mseri #778)
 
 ## v4.0.0 (2021-03-24)
 
diff --git a/cohttp-lwt-unix/test/test_parser.ml b/cohttp-lwt-unix/test/test_parser.ml
index 025627f0d6..3bf2692f78 100644
--- a/cohttp-lwt-unix/test/test_parser.ml
+++ b/cohttp-lwt-unix/test/test_parser.ml
@@ -246,7 +246,7 @@ let make_simple_req () =
   let open Cohttp in
   let open Cohttp_lwt_unix in
   let expected =
-    "POST /foo/bar HTTP/1.1\r\nfoo: bar\r\nhost: localhost\r\nuser-agent: "
+    "POST /foo/bar HTTP/1.1\r\nFoo: bar\r\nhost: localhost\r\nuser-agent: "
     ^ user_agent
     ^ "\r\ntransfer-encoding: chunked\r\n\r\n6\r\nfoobar\r\n0\r\n\r\n"
   in
diff --git a/cohttp/fuzz/fuzz_header.ml b/cohttp/fuzz/fuzz_header.ml
index d53b743c04..e5640f1d49 100644
--- a/cohttp/fuzz/fuzz_header.ml
+++ b/cohttp/fuzz/fuzz_header.ml
@@ -189,10 +189,9 @@ let is_empty_test () =
 let init_with_test () =
   Crowbar.(
     (* FS *)
-    (* forall k v. to_list (init_with k v) = [String.lowercase k, v] *)
+    (* forall k v. to_list (init_with k v) = [k, v] *)
     add_test ~name:"[init_list k v] is [k, v]" [ header_name_gen; word_gen ]
-      (fun k v ->
-        check_eq H.(to_list (init_with k v)) [ (String.lowercase_ascii k, v) ]))
+      (fun k v -> check_eq H.(to_list (init_with k v)) [ (k, v) ]))
 
 let mem_test () =
   Crowbar.(
@@ -201,12 +200,16 @@ let mem_test () =
     add_test ~name:"[mem h k] on an empty header is always false"
       [ header_name_gen ] (fun k -> check_eq false H.(mem (init ()) k));
     (* SI *)
-    (* forall h, k. H.mem h k = List.(mem_assoc k (H.to_list h)) *)
+    (* forall h, k. H.mem h k = List.(mem_assoc (String.lowercase_ascii x) (List.map (fun (k, v) -> String.lowercase_ascii k, v) (H.to_list h))) *)
     add_test ~name:"Header.mem has the same behavior than List.mem_assoc"
       [ headers_gen; header_name_gen ] (fun h k ->
         check_eq
           H.(mem h k)
-          List.(mem_assoc (String.lowercase_ascii k) (H.to_list h))))
+          List.(
+            mem_assoc (String.lowercase_ascii k)
+              (List.map
+                 (fun (k, v) -> (String.lowercase_ascii k, v))
+                 (H.to_list h)))))
 
 let add_test () =
   Crowbar.(
@@ -220,9 +223,7 @@ let add_test () =
     (* forall h, k, v. to_list (add h k v) = to_list h @ [lowercase k, v] *)
       ~name:"[add] adds a value at the header end"
       [ headers_gen; header_name_gen; word_gen ] (fun h k v ->
-        check_eq
-          (H.to_list h @ [ (String.lowercase_ascii k, v) ])
-          H.(to_list (add h k v))))
+        check_eq (H.to_list h @ [ (k, v) ]) H.(to_list (add h k v))))
 
 let to_list_of_list_test () =
   Crowbar.(
diff --git a/cohttp/src/header.ml b/cohttp/src/header.ml
index 2f3166e4fe..f4995c07ed 100644
--- a/cohttp/src/header.ml
+++ b/cohttp/src/header.ml
@@ -16,36 +16,37 @@
  *
   }}}*)
 
-module LString : sig
-  type t
-
-  val of_string : string -> t
-  val to_string : t -> string
-  val compare : t -> t -> int
-end = struct
-  type t = string
-
-  let of_string x = String.lowercase_ascii x
-  let to_string x = x
-  let compare a b = String.compare a b
-end
-
-type t = (LString.t * string) list
+let caseless_equal a b =
+  if a == b then true
+  else
+    let len = String.length a in
+    len = String.length b
+    &&
+    let stop = ref false in
+    let idx = ref 0 in
+    while (not !stop) && !idx < len do
+      let c1 = String.unsafe_get a !idx in
+      let c2 = String.unsafe_get b !idx in
+      if Char.lowercase_ascii c1 <> Char.lowercase_ascii c2 then stop := true;
+      incr idx
+    done;
+    not !stop
+
+type t = (string * string) list
 
 let compare = Stdlib.compare
 let init () = []
 let is_empty = function [] -> true | _ -> false
-let init_with k v = [ (LString.of_string k, v) ]
+let init_with k v = [ (k, v) ]
 
 let mem h k =
-  let k = LString.of_string k in
   let rec loop = function
     | [] -> false
-    | (k', _) :: h' -> if LString.compare k k' = 0 then true else loop h'
+    | (k', _) :: h' -> if caseless_equal k k' then true else loop h'
   in
   loop h
 
-let add h k v : t = (LString.of_string k, v) :: h
+let add h k v : t = (k, v) :: h
 let add_list h l = List.fold_left (fun h (k, v) -> add h k v) h l
 let add_multi h k l = List.fold_left (fun h v -> add h k v) h l
 
@@ -58,11 +59,10 @@ let add_opt_unless_exists h k v =
   match h with None -> init_with k v | Some h -> add_unless_exists h k v
 
 let get h k =
-  let k = LString.of_string k in
   let rec loop h =
     match h with
     | [] -> None
-    | (k', v) :: h' -> if LString.compare k k' = 0 then Some v else loop h'
+    | (k', v) :: h' -> if caseless_equal k k' then Some v else loop h'
   in
   loop h
 
@@ -71,36 +71,32 @@ let get_multi (h : t) (k : string) =
     match h with
     | [] -> acc
     | (k', v) :: h' ->
-        if LString.compare (LString.of_string k) k' = 0 then loop h' (v :: acc)
-        else loop h' acc
+        if caseless_equal k k' then loop h' (v :: acc) else loop h' acc
   in
   loop h []
 
 let remove h k =
-  let k = LString.of_string k in
   let rec loop seen = function
     | [] -> if seen then [] else raise Not_found
-    | (k', _) :: h when LString.compare k k' = 0 -> loop true h
+    | (k', _) :: h when caseless_equal k k' -> loop true h
     | x :: h -> x :: loop seen h
   in
   try loop false h with Not_found -> h
 
 let remove_last h k =
-  let k = LString.of_string k in
   let rec loop seen = function
     | [] -> raise Not_found
-    | (k', _) :: h when LString.compare k k' = 0 -> h
+    | (k', _) :: h when caseless_equal k k' -> h
     | x :: h -> x :: loop seen h
   in
   try loop false h with Not_found -> h
 
 let replace_ last h k v =
-  let k' = LString.of_string k in
   let rec loop seen = function
     | [] -> if seen then [] else raise Not_found
-    | (k'', _) :: h when LString.compare k' k'' = 0 ->
+    | (k'', _) :: h when caseless_equal k k'' ->
         if last then (k'', v) :: h
-        else if not seen then (k', v) :: loop true h
+        else if not seen then (k, v) :: loop true h
         else loop seen h
     | x :: h -> x :: loop seen h
   in
@@ -129,33 +125,26 @@ let update_all h k f =
 let map (f : string -> string -> string) (h : t) : t =
   List.map
     (fun (k, v) ->
-      let vs' = f (LString.to_string k) v in
+      let vs' = f k v in
       (k, vs'))
     h
 
 let iter (f : string -> string -> unit) (h : t) : unit =
-  List.iter (fun (k, v) -> f (LString.to_string k) v) h
+  List.iter (fun (k, v) -> f k v) h
 
 let fold (f : string -> string -> 'a -> 'a) (h : t) (init : 'a) : 'a =
-  List.fold_left (fun acc (k, v) -> f (LString.to_string k) v acc) init h
-
-let of_list h =
-  List.fold_left (fun acc (k, v) -> (LString.of_string k, v) :: acc) [] h
+  List.fold_left (fun acc (k, v) -> f k v acc) init h
 
-let to_list h =
-  List.fold_left (fun acc (k, v) -> (LString.to_string k, v) :: acc) [] h
+let of_list h = List.rev h
+let to_list h = List.rev h
 
 let to_lines (h : t) =
   let header_line k v = Printf.sprintf "%s: %s\r\n" k v in
-  List.fold_left
-    (fun acc (k, v) -> header_line (LString.to_string k) v :: acc)
-    [] h
+  List.fold_left (fun acc (k, v) -> header_line k v :: acc) [] h
 
 let to_frames h =
   let to_frame k v = Printf.sprintf "%s: %s" k v in
-  List.fold_left
-    (fun acc (k, v) -> to_frame (LString.to_string k) v :: acc)
-    [] h
+  List.fold_left (fun acc (k, v) -> to_frame k v :: acc) [] h
 
 let to_string h =
   let b = Buffer.create 128 in
@@ -169,48 +158,47 @@ let to_string h =
   Buffer.contents b
 
 let headers_with_list_values =
-  Array.map LString.of_string
-    [|
-      "accept";
-      "accept-charset";
-      "accept-encoding";
-      "accept-language";
-      "accept-ranges";
-      "allow";
-      "cache-control";
-      "connection";
-      "content-encoding";
-      "content-language";
-      "expect";
-      "if-match";
-      "if-none-match";
-      "link";
-      "pragma";
-      "proxy-authenticate";
-      "te";
-      "trailer";
-      "transfer-encoding";
-      "upgrade";
-      "vary";
-      "via";
-      "warning";
-      "www-authenticate";
-    |]
+  [|
+    "accept";
+    "accept-charset";
+    "accept-encoding";
+    "accept-language";
+    "accept-ranges";
+    "allow";
+    "cache-control";
+    "connection";
+    "content-encoding";
+    "content-language";
+    "expect";
+    "if-match";
+    "if-none-match";
+    "link";
+    "pragma";
+    "proxy-authenticate";
+    "te";
+    "trailer";
+    "transfer-encoding";
+    "upgrade";
+    "vary";
+    "via";
+    "warning";
+    "www-authenticate";
+  |]
 
 let is_header_with_list_value =
   let tbl = Hashtbl.create (Array.length headers_with_list_values) in
   headers_with_list_values |> Array.iter (fun h -> Hashtbl.add tbl h ());
   fun h -> Hashtbl.mem tbl h
 
-let is_set_cookie k = LString.(compare k (of_string "set-cookie"))
+let is_set_cookie k = caseless_equal k "set-cookie"
 
 (* set-cookie is an exception according to
    {{:https://tools.ietf.org/html/rfc7230#section-3.2.2}
-   RFC7230§3.2.2} and can appear multiple times in a response message.
+    RFC7230§3.2.2} and can appear multiple times in a response message.
 *)
 let clean_dup (h : t) : t =
   let add h k v =
-    if is_set_cookie k = 0 then (k, v) :: h
+    if is_set_cookie k then (k, v) :: h
     else
       let to_add = ref false in
       let rec loop = function
@@ -218,7 +206,7 @@ let clean_dup (h : t) : t =
             to_add := true;
             []
         | (k', v') :: hs ->
-            if LString.compare k k' = 0 then
+            if caseless_equal k k' then
               if is_header_with_list_value k then (k, v' ^ "," ^ v) :: hs
               else (
                 to_add := true;
@@ -231,8 +219,7 @@ let clean_dup (h : t) : t =
   List.rev h |> List.fold_left (fun acc (k, v) -> add acc k v) []
 
 let get_multi_concat ?(list_value_only = false) h k : string option =
-  if (not list_value_only) || is_header_with_list_value (LString.of_string k)
-  then
+  if (not list_value_only) || is_header_with_list_value k then
     let vs = get_multi h k in
     match vs with [] -> None | _ -> Some (String.concat "," vs)
   else get h k
diff --git a/cohttp/src/header.mli b/cohttp/src/header.mli
index a3edb4ec9e..38d6823c99 100644
--- a/cohttp/src/header.mli
+++ b/cohttp/src/header.mli
@@ -32,7 +32,8 @@ val of_list : (string * string) list -> t
     is true with case insensitive comparison. *)
 
 val to_list : t -> (string * string) list
-(** [to_list h] converts HTTP headers [h] to a list. Order is preserved.
+(** [to_list h] converts HTTP headers [h] to a list. Order and case is
+    preserved.
 
     {e Invariant (with case insensitive comparison):} [to_list (of_list l) = l] *)
 
diff --git a/cohttp/test/unitary_test_header.ml b/cohttp/test/unitary_test_header.ml
index d3421e8764..a47b879544 100644
--- a/cohttp/test/unitary_test_header.ml
+++ b/cohttp/test/unitary_test_header.ml
@@ -58,7 +58,7 @@ let is_empty_tests () =
 
 let init_with_tests () =
   aessl "init_with k v"
-    [ ("transfer-encoding", "chunked") ]
+    [ ("traNsfer-eNcoding", "chunked") ]
     H.(to_list (init_with "traNsfer-eNcoding" "chunked"))
 
 let mem_tests () =