This is a big project. It may be a little before I can get it to build.

What have you tried debugging wise? Have you tested latency? Ping?

EDIT
I'm trying to build this in CB, but it throws a ton of warnings, some of which you can't ignore. Until you fix the warnings, I can't really help. A CMakeLists.txt file would also go a long way towards getting other people to help you.

Edgar Reynaldo here is a CMakeLists attached.
Might want to remove lines in the CMakeLists file as I forgot to do it
../egf/*.hpp -> All lines named like this, remove them, as they are not actual files in the zipped project I attached.
Note: It probably won't run in Codeblocks anyway, since I ported it from Codeblocks to Visual Studio and had to make certain changes for Visual C compiler to pick it up. Not sure if GCC can pick up these changes as valid code.

Chris Katko, no that is not the game. That's a game I'm developing from scratch inspired from Endless Online. Although the client in the video is not developed by me.
The program in question here in this thread is attached, which is supposed to be an identical clone of the official client 0.28.

1) what is "lag" in this scenario? Actual delay between control input and output? Or reduced frame rate?
Extreme reduced frame rate + lag with controls. Hit a key, it starts walking 2 seconds after + everything has reduced frame rate, even more when more entities on the game.
The lag + input lag decreases as I walk on a small empty map.

2) have you used a profiler?
Dunno what that is really, also not sure, since I didn't write the code, but I have looked into it and I can't really figure out what's causing the lag.

[edit]

3) Are you drawing all the isometric tiles even if they're outside the screen?
I believe the code writes only the tiles that are in the screen.

Sorry no, a guy named Sausage made that CMakeLists file and I have no experience with CMake. I can ask him, but he might or might not give it to me, idk.

Edit: Attached a cmake file

I compiled it, ran it and debugged the issues. Your game has many issues.

The primary cause of slowness is how you use your loading thread. In your loading thread you load bitmaps as memory bitmaps (good), but in your drawing thread you never convert them to video bitmaps, which means you're drawing using memory bitmaps only which is very slow.

That's relatively easy to fix for bitmaps at least, inside your Bitmap::Loaded function, you can do:

if (!(b && (flag & ALLEGRO_VIDEO_BITMAP)))
{
  al_convert_bitmap(bitmap.get());
}

And that'll convert the bitmaps. You also need to do this for fonts, which isn't as easy. I suggest loading the fonts as bitmaps in the loading thread, and then using al_convert_bitmap + al_grab_font_from_bitmap in the drawing thread. Maybe we (Allegro devs) should add a al_convert_bitmap_font to make this easier. See this example for how to do loading threads correctly: ex_loading_thread.

There may be more memory bitmaps, but those were the main ones I saw. There are many more issues other than this though!

First, I noticed you're calling al_create_bitmap every frame when drawing characters, you'll want to not do that: pre-create your bitmaps on-load.

Second, even if you convert everything to video bitmaps, it'll still be somewhat slow unless you use al_hold_bitmap_drawing around your drawing for loops:

al_hold_bitmap_drawing(true);
for (...)
  DrawTinted()
al_hold_bitmap_drawing(false);

I added al_convert_bitmap(bitmap.get());
Still very choppy.
You can try and log in with username: testeoplay password: testtest
to check it in-game, this will make it easier for you to see what's up

I tried using al_convert_memory_bitmaps(); in the loaded function, to convert them all to proper bitmaps, not working either.