Heavy lag on fc 26

Hi everyone, i don't understand why my fc is lagging so much if my is optimized for these games and much more. It's not making any sense for me it's like the game slows down at moments and then it's clear and good for a few moments. It's a cycle that makes the game unplayable and i can't seem to fix the issue, even when I lower the graphics settings. If anyone could help me, it would be much appreciated thank you.