Its becouse ( i guess ) you are running windows 10 by default without changing any settings on it. Windows 10 comes with so much trash running in background that no matter how strong your PC is , over time it will start lagging and clogging .
I know this becouse i helped alot of ppl inthis community. I can help you personally but if you are sceptical since you probably dont know me , i will send you link to youtube video that will guide you more or less what you need to do.
there are still few tricks that you can do but im too lazy to write here.
It seems that the majority, if not everyone, is suffering from massive decreases in FPS. For example, I played a match last week where my FPS was constantly 40-50 on the Port skirmish map (same settings as before). Before that, I would get a steady 144 on lowest settings with resolution about 90%. I hope this issue will be addressed for the sake of being able to play this game competitively. @Callum@AVRC