Fullscreen Quality "High" and "Replace Solid Colour" effects is lowering FPS (270)

If your game's performance is bottlenecked on GPU work, then using high quality fullscreen mode will require even more GPU work, and so possibly slow down the game. For games bottlenecked on GPU work, that's not a bug, it's just your game design reaching the limits of the hardware.

I couldn't find much of a difference in my project. Maybe it's +10fps if I set it to low, but considering that my game runs at ~550fps in unlimited mode it doesn't seem to have all that much impact. That said, I only use one effect (multiply on a layer) so it probably has something to do with the effects. If you can share the project or maybe a file that demonstrates it I can try it out on my PC.

I do know that effects tend to have quite the impact on fps in general, so I avoid them where I can and use them sparingly. I also remember reading somewhere that it often is better to apply the effect to an entire layer if possible. Say you want to color 100 sprites blue with an effect. Adding the effect to all 100 sprites individually adds a lot of overhead, where if you just plop the sprites on a layer with the effect it just has to apply it once.

EDIT: Yeah it's in the documentation: construct.net/en/make-games/manuals/construct-3/project-primitives/objects/effects

So I recommend trying that.