The problem might be server side, yes. The game is probably on a higher than average tickrate to improve latency between client-server but thats also overloads the server CPU and network, that’s why the frame rate drops. It was also proven that the game doesn’t utilize your GPU to load ■■■■ which is a factor that will contribute a lot with frame rate, that’s why lowering the graphics improve FPS but it will still drop to abyssmal levels when a lot of garbage is happening at the same time. Watch any video of the game on youtube and you will see that, on every single video, the frame rate is ass.
Lag is a completly different issue, lag happens when client-server communications loses packets, which just make the game “stop”(nothing moves) but packets are still trying to be sent so when the server receives it, it receives everything at the same time so, if you killed something, moved, sent a message or anything else during the time you were “off”, after the connection establishes the server will receive all the packets you tried to sent at the same time so that’s why everything you did on a spam of 3 seconds happens in 1.