diff --git a/compiler/ccgexprs.nim b/compiler/ccgexprs.nim
index 0b9958ee4d41..ec54081a23e3 100644
--- a/compiler/ccgexprs.nim
+++ b/compiler/ccgexprs.nim
@@ -1194,9 +1194,11 @@ proc genSeqElemAppend(p: BProc, e: PNode, d: var TLoc) =
 proc genReset(p: BProc, n: PNode) =
   var a: TLoc
   initLocExpr(p, n[1], a)
-  linefmt(p, cpsStmts, "#genericReset((void*)$1, $2);$n",
-          [addrLoc(p.config, a),
-          genTypeInfo(p.module, skipTypes(a.t, {tyVar}), n.info)])
+  specializeReset(p, a)
+  when false:
+    linefmt(p, cpsStmts, "#genericReset((void*)$1, $2);$n",
+            [addrLoc(p.config, a),
+            genTypeInfo(p.module, skipTypes(a.t, {tyVar}), n.info)])
 
 proc genDefault(p: BProc; n: PNode; d: var TLoc) =
   if d.k == locNone: getTemp(p, n.typ, d, needsInit=true)
@@ -2793,6 +2795,21 @@ proc getDefaultValue(p: BProc; typ: PType; info: TLineInfo): Rope =
   else:
     globalError(p.config, info, "cannot create null element for: " & $t.kind)
 
+proc caseObjDefaultBranch(obj: PNode; branch: Int128): int =
+  for i in 1 ..< obj.len:
+    for j in 0 .. obj[i].len - 2:
+      if obj[i][j].kind == nkRange:
+        let x = getOrdValue(obj[i][j][0])
+        let y = getOrdValue(obj[i][j][1])
+        if branch >= x and branch <= y:
+          return i
+      elif getOrdValue(obj[i][j]) == branch:
+        return i
+    if obj[i].len == 1:
+      # else branch
+      return i
+  assert(false, "unreachable")
+
 proc getNullValueAux(p: BProc; t: PType; obj, constOrNil: PNode,
                      result: var Rope; count: var int;
                      isConst: bool, info: TLineInfo) =
@@ -2815,31 +2832,14 @@ proc getNullValueAux(p: BProc; t: PType; obj, constOrNil: PNode,
           branch = getOrdValue(constOrNil[i])
           break
 
-    var selectedBranch = -1
-    block branchSelection:
-      for i in 1 ..< obj.len:
-        for j in 0 .. obj[i].len - 2:
-          if obj[i][j].kind == nkRange:
-              let x = getOrdValue(obj[i][j][0])
-              let y = getOrdValue(obj[i][j][1])
-              if branch >= x and branch <= y:
-                selectedBranch = i
-                break branchSelection
-          elif getOrdValue(obj[i][j]) == branch:
-            selectedBranch = i
-            break branchSelection
-        if obj[i].len == 1:
-          # else branch
-          selectedBranch = i
-    assert(selectedBranch >= 1)
-
+    let selectedBranch = caseObjDefaultBranch(obj, branch)
     result.add "{"
     var countB = 0
     let b = lastSon(obj[selectedBranch])
     # designated initilization is the only way to init non first element of unions
     # branches are allowed to have no members (b.len == 0), in this case they don't need initializer
-    if  b.kind == nkRecList and b.len > 0:
-      result.add "._" &  mangleRecFieldName(p.module, obj[0].sym) & "_" & $selectedBranch & " = {"
+    if b.kind == nkRecList and b.len > 0:
+      result.add "._" & mangleRecFieldName(p.module, obj[0].sym) & "_" & $selectedBranch & " = {"
       getNullValueAux(p, t,  b, constOrNil, result, countB, isConst, info)
       result.add "}"
     elif b.kind == nkSym:
diff --git a/compiler/ccgreset.nim b/compiler/ccgreset.nim
new file mode 100644
index 000000000000..ef1505f5799d
--- /dev/null
+++ b/compiler/ccgreset.nim
@@ -0,0 +1,94 @@
+#
+#
+#           The Nim Compiler
+#        (c) Copyright 2020 Andreas Rumpf
+#
+#    See the file "copying.txt", included in this
+#    distribution, for details about the copyright.
+#
+
+# included from cgen.nim
+
+## Code specialization instead of the old, incredibly slow 'genericReset'
+## implementation.
+
+proc specializeResetT(p: BProc, accessor: Rope, typ: PType)
+
+proc specializeResetN(p: BProc, accessor: Rope, n: PNode;
+                     typ: PType) =
+  if n == nil: return
+  case n.kind
+  of nkRecList:
+    for i in 0..<n.len:
+      specializeResetN(p, accessor, n[i], typ)
+  of nkRecCase:
+    if (n[0].kind != nkSym): internalError(p.config, n.info, "specializeResetN")
+    let disc = n[0].sym
+    if disc.loc.r == nil: fillObjectFields(p.module, typ)
+    if disc.loc.t == nil:
+      internalError(p.config, n.info, "specializeResetN()")
+    lineF(p, cpsStmts, "switch ($1.$2) {$n", [accessor, disc.loc.r])
+    for i in 1..<n.len:
+      let branch = n[i]
+      assert branch.kind in {nkOfBranch, nkElse}
+      if branch.kind == nkOfBranch:
+        genCaseRange(p, branch)
+      else:
+        lineF(p, cpsStmts, "default:$n", [])
+      specializeResetN(p, accessor, lastSon(branch), typ)
+      lineF(p, cpsStmts, "break;$n", [])
+    lineF(p, cpsStmts, "} $n", [])
+    specializeResetT(p, "$1.$2" % [accessor, disc.loc.r], disc.loc.t)
+  of nkSym:
+    let field = n.sym
+    if field.typ.kind == tyVoid: return
+    if field.loc.r == nil: fillObjectFields(p.module, typ)
+    if field.loc.t == nil:
+      internalError(p.config, n.info, "specializeResetN()")
+    specializeResetT(p, "$1.$2" % [accessor, field.loc.r], field.loc.t)
+  else: internalError(p.config, n.info, "specializeResetN()")
+
+proc specializeResetT(p: BProc, accessor: Rope, typ: PType) =
+  if typ == nil: return
+
+  case typ.kind
+  of tyGenericInst, tyGenericBody, tyTypeDesc, tyAlias, tyDistinct, tyInferred,
+     tySink, tyOwned:
+    specializeResetT(p, accessor, lastSon(typ))
+  of tyArray:
+    let arraySize = lengthOrd(p.config, typ[0])
+    var i: TLoc
+    getTemp(p, getSysType(p.module.g.graph, unknownLineInfo, tyInt), i)
+    linefmt(p, cpsStmts, "for ($1 = 0; $1 < $2; $1++) {$n",
+            [i.r, arraySize])
+    specializeResetT(p, ropecg(p.module, "$1[$2]", [accessor, i.r]), typ[1])
+    lineF(p, cpsStmts, "}$n", [])
+  of tyObject:
+    for i in 0..<typ.len:
+      var x = typ[i]
+      if x != nil: x = x.skipTypes(skipPtrs)
+      specializeResetT(p, accessor.parentObj(p.module), x)
+    if typ.n != nil: specializeResetN(p, accessor, typ.n, typ)
+  of tyTuple:
+    let typ = getUniqueType(typ)
+    for i in 0..<typ.len:
+      specializeResetT(p, ropecg(p.module, "$1.Field$2", [accessor, i]), typ[i])
+
+  of tyString, tyRef, tySequence:
+    lineCg(p, cpsStmts, "#unsureAsgnRef((void**)&$1, NIM_NIL);$n", [accessor])
+
+  of tyProc:
+    if typ.callConv == ccClosure:
+      lineCg(p, cpsStmts, "#unsureAsgnRef((void**)&$1.ClE_0, NIM_NIL);$n", [accessor])
+      lineCg(p, cpsStmts, "$1.ClP_0 = NIM_NIL;$n", [accessor])
+    else:
+      lineCg(p, cpsStmts, "$1 = NIM_NIL;$n", [accessor])
+  of tyChar, tyBool, tyEnum, tyInt..tyUInt64:
+    lineCg(p, cpsStmts, "$1 = 0;$n", [accessor])
+  of tyCString, tyPointer, tyPtr, tyVar, tyLent:
+    lineCg(p, cpsStmts, "$1 = NIM_NIL;$n", [accessor])
+  else:
+    discard
+
+proc specializeReset(p: BProc, a: TLoc) =
+  specializeResetT(p, rdLoc(a), a.t)
diff --git a/compiler/cgen.nim b/compiler/cgen.nim
index c32e86a8894d..8fa824fcc7e6 100644
--- a/compiler/cgen.nim
+++ b/compiler/cgen.nim
@@ -390,6 +390,8 @@ proc isComplexValueType(t: PType): bool {.inline.} =
   result = t.kind in {tyArray, tySet, tyTuple, tyObject} or
     (t.kind == tyProc and t.callConv == ccClosure)
 
+include ccgreset
+
 proc resetLoc(p: BProc, loc: var TLoc) =
   let containsGcRef = optSeqDestructors notin p.config.globalOptions and containsGarbageCollectedRef(loc.t)
   let typ = skipTypes(loc.t, abstractVarRange)
@@ -407,8 +409,10 @@ proc resetLoc(p: BProc, loc: var TLoc) =
       linefmt(p, cpsStmts, "$1 = 0;$n", [rdLoc(loc)])
   else:
     if loc.storage != OnStack and containsGcRef:
-      linefmt(p, cpsStmts, "#genericReset((void*)$1, $2);$n",
-              [addrLoc(p.config, loc), genTypeInfo(p.module, loc.t, loc.lode.info)])
+      specializeReset(p, loc)
+      when false:
+        linefmt(p, cpsStmts, "#genericReset((void*)$1, $2);$n",
+                [addrLoc(p.config, loc), genTypeInfo(p.module, loc.t, loc.lode.info)])
       # XXX: generated reset procs should not touch the m_type
       # field, so disabling this should be safe:
       genObjectInit(p, cpsStmts, loc.t, loc, constructObj)
diff --git a/lib/system/gc_ms.nim b/lib/system/gc_ms.nim
index 0b9745f1e74a..6683b9e349e9 100644
--- a/lib/system/gc_ms.nim
+++ b/lib/system/gc_ms.nim
@@ -104,7 +104,7 @@ proc extGetCellType(c: pointer): PNimType {.compilerproc.} =
   # used for code generation concerning debugging
   result = usrToCell(c).typ
 
-proc unsureAsgnRef(dest: PPointer, src: pointer) {.inline.} =
+proc unsureAsgnRef(dest: PPointer, src: pointer) {.inline, compilerproc.} =
   dest[] = src
 
 proc internRefcount(p: pointer): int {.exportc: "getRefcount".} =