Low FPS on 2D browser based game?

Low FPS on 2D browser based game?

I'm having trouble getting a 2D browser based game to work satisfactorily.
It has nothing but 2d images (sprites) which should be displayed on their
correct positions on the screen.

I've started with Processing.js (that uses Canvas): 30 fps on my computer,
16 fps on my friends. I then migrated to Pixi.js that uses WebGL: 200 fps
on my computer, but 8 fps on my friends (!?). This was surprising, because
my friends computer can run Counter Strike (which is 3D) and a similar 2D
game based on OpenGL (Tibia) at 40+ FPS. With just one sprite the FPS goes
up to 50, so it is not the rest of the program that is to blame.
Is this a limitation of WebGL itself? Should I just migrate back to a
standard C client?
The link for the game is http://viclib.com/lintae .