Your FPS depends on the amount of objects/ things for the GPU to load and display. For example I normally get around a 100 FPS in Overwatch on High settings. But, when there are thousands of particles and ULTS etc my fps drops to about 60-70 sometimes even lower. I assume that you get around 240 when you are having a massive build battle with multiple people and all that fun stuff. I also assume that you get 500+ when the match is just starting and there is not much to render. Anyways, its not your hardware in fact 240 is more than enough for top tier competitive play, its just as time goes on in a build battle like that your GPU has more and more things to load and so you get less FPS