diff --git a/changelog.d/5970.docker b/changelog.d/5970.docker
new file mode 100644
index 000000000000..c9d04da9cd53
--- /dev/null
+++ b/changelog.d/5970.docker
@@ -0,0 +1 @@
+Avoid changing UID/GID if they are already correct.
diff --git a/docker/start.py b/docker/start.py
index 40a861f200c1..260f2d9943ed 100755
--- a/docker/start.py
+++ b/docker/start.py
@@ -41,8 +41,8 @@ def generate_config_from_template(config_dir, config_path, environ, ownership):
         config_dir (str): where to put generated config files
         config_path (str): where to put the main config file
         environ (dict): environment dictionary
-        ownership (str): "<user>:<group>" string which will be used to set
-            ownership of the generated configs
+        ownership (str|None): "<user>:<group>" string which will be used to set
+            ownership of the generated configs. If None, ownership will not change.
     """
     for v in ("SYNAPSE_SERVER_NAME", "SYNAPSE_REPORT_STATS"):
         if v not in environ:
@@ -105,24 +105,24 @@ def generate_config_from_template(config_dir, config_path, environ, ownership):
     log("Generating log config file " + log_config_file)
     convert("/conf/log.config", log_config_file, environ)
 
-    subprocess.check_output(["chown", "-R", ownership, "/data"])
-
     # Hopefully we already have a signing key, but generate one if not.
-    subprocess.check_output(
-        [
-            "su-exec",
-            ownership,
-            "python",
-            "-m",
-            "synapse.app.homeserver",
-            "--config-path",
-            config_path,
-            # tell synapse to put generated keys in /data rather than /compiled
-            "--keys-directory",
-            config_dir,
-            "--generate-keys",
-        ]
-    )
+    args = [
+        "python",
+        "-m",
+        "synapse.app.homeserver",
+        "--config-path",
+        config_path,
+        # tell synapse to put generated keys in /data rather than /compiled
+        "--keys-directory",
+        config_dir,
+        "--generate-keys",
+    ]
+
+    if ownership is not None:
+        subprocess.check_output(["chown", "-R", ownership, "/data"])
+        args = ["su-exec", ownership] + args
+
+    subprocess.check_output(args)
 
 
 def run_generate_config(environ, ownership):
@@ -130,7 +130,7 @@ def run_generate_config(environ, ownership):
 
     Args:
         environ (dict): env var dict
-        ownership (str): "userid:groupid" arg for chmod
+        ownership (str|None): "userid:groupid" arg for chmod. If None, ownership will not change.
 
     Never returns.
     """
@@ -149,9 +149,6 @@ def run_generate_config(environ, ownership):
         log("Creating log config %s" % (log_config_file,))
         convert("/conf/log.config", log_config_file, environ)
 
-    # make sure that synapse has perms to write to the data dir.
-    subprocess.check_output(["chown", ownership, data_dir])
-
     args = [
         "python",
         "-m",
@@ -170,12 +167,33 @@ def run_generate_config(environ, ownership):
         "--open-private-ports",
     ]
     # log("running %s" % (args, ))
-    os.execv("/usr/local/bin/python", args)
+
+    if ownership is not None:
+        args = ["su-exec", ownership] + args
+        os.execv("/sbin/su-exec", args)
+
+        # make sure that synapse has perms to write to the data dir.
+        subprocess.check_output(["chown", ownership, data_dir])
+    else:
+        os.execv("/usr/local/bin/python", args)
 
 
 def main(args, environ):
     mode = args[1] if len(args) > 1 else None
-    ownership = "{}:{}".format(environ.get("UID", 991), environ.get("GID", 991))
+    desired_uid = int(environ.get("UID", "991"))
+    desired_gid = int(environ.get("GID", "991"))
+    if (desired_uid == os.getuid()) and (desired_gid == os.getgid()):
+        ownership = None
+    else:
+        ownership = "{}:{}".format(desired_uid, desired_gid)
+
+    log(
+        "Container running as UserID %s:%s, ENV (or defaults) requests %s:%s"
+        % (os.getuid(), os.getgid(), desired_uid, desired_gid)
+    )
+
+    if ownership is None:
+        log("Will not perform chmod/su-exec as UserID already matches request")
 
     # In generate mode, generate a configuration and missing keys, then exit
     if mode == "generate":
@@ -227,16 +245,12 @@ def main(args, environ):
 
     log("Starting synapse with config file " + config_path)
 
-    args = [
-        "su-exec",
-        ownership,
-        "python",
-        "-m",
-        "synapse.app.homeserver",
-        "--config-path",
-        config_path,
-    ]
-    os.execv("/sbin/su-exec", args)
+    args = ["python", "-m", "synapse.app.homeserver", "--config-path", config_path]
+    if ownership is not None:
+        args = ["su-exec", ownership] + args
+        os.execv("/sbin/su-exec", args)
+    else:
+        os.execv("/usr/local/bin/python", args)
 
 
 if __name__ == "__main__":