As many of you don't seem to understand my issue, here is a graphical explanation.
As you can see, in LabyMod 1.8.9 I get 60 fps locked (don't mind the fps there it's because of shaders).
On 1.15.2 Optifine I get a lot more FPS.
Also fullscreen mode on 1.8.9 labymod doesn't hide top bar shell.