net.http: Send entire HTTP request header as one write When opportunistic writes are enabled this reduces the number of syscalls and TCP packets sent on the wire. Experiments with TCP Fast Open made this even more obvious. That table trick probably wasn't as efficient. Lua generates bytecode for a table with zero array slots and space for two entries in the hash part, plus code to set [2] and [4]. I didn't verify but I suspect it would have had to resize the table when setting [1] and [3], although probably only once. Concatenating the strings directly in Lua is easier to read and involves no extra table or function call.

--- a/net/http.lua	Thu Jul 08 17:57:44 2021 +0200
+++ b/net/http.lua	Thu Jul 08 18:21:59 2021 +0200
@@ -164,13 +164,11 @@
 		t_insert(request_line, 4, "?"..req.query);
 	end
 
+	for k, v in pairs(req.headers) do
+		t_insert(request_line, k .. ": " .. v .. "\r\n");
+	end
+	t_insert(request_line, "\r\n")
 	conn:write(t_concat(request_line));
-	local t = { [2] = ": ", [4] = "\r\n" };
-	for k, v in pairs(req.headers) do
-		t[1], t[3] = k, v;
-		conn:write(t_concat(t));
-	end
-	conn:write("\r\n");
 
 	if req.body then
 		conn:write(req.body);