> But then, why do you think it might be the GPU in my case?
Mostly because the only game you mentioned that I'm familiar with (SuperTux) is a game that is going to have its performance heavily influenced by how good the GPU is. Despite being 2-D, SuperTux uses OpenGL and is actually quite heavy. Also because you mentioned that this problem only occurs when you play the games at the full resolution of your monitor.
One thing you can check is whether or not SuperTux runs better with the "--renderer sdl" command-line option. If this substantially improves your performance, that would boost my confidence in your GPU being the bottleneck.
