Rule of thumb when posting performance issues - ALWAYS attach a profiler capture. To get it, press Ctrl+F6 to display the profiler, and pick Dump in the menu (pick number of frames you want to save data for - 32 is usually enough for continuous performance issues). It will save an HTML file to C:\Users\yourusername - you should attach that file to the post.
Specifically in this case, you should take the capture when you start playing the game (while performance is good), and when the performance becomes bad. I don’t know if the profiling captures will help but they definitely won’t hurt… (and make sure to not have video recording enabled - it interferes with performance enough to obscure the possible causes)