Laggs despite good FPS?

I recently assembled a PC. Have Battlefield 1, GTA 5, etc. Tested running without problems but eg. In Fortnite I have only in the first round when I land strong slumps in the next rounds I have no problems. In Rocket League I also have slight stuttering which is pretty annoying. Would anyone have an idea what the problem could be?

Here are my specs:

Motherboard: MSI B350M Pro-VDH

CPU: Amd Ryzen 5 1600

GPU: Nvidia GTX 1050ti

RAM: 2x4 GB 2400MHz

Power adapter: 500 watts

That's probably the CPU that you rummuckt.

What do you mean exactly?