I've had this issue since the beginning of the game, I've tried many solutions and none of them worked, the only solution when that happened to me was to either close the game or just quit the match and quit ultimate team an relogged in, but in a reddit forum I read a crazy solution that worked for me. The user that came out with the solution is Necessary-Tie6791, I quote:
"I FOUND A WEIRD FIX ANYONE SUFFERING FROM FPS DROP, GPU GOING 100% AT START OR HALFTIME, whenever it happens minimize fc and open any other game on ur pc and wait until the game game loads up to the menu and quit the game then open fc 26 and the problem should be gone fps will be normal again
Keep borderless in display settings in fc 26 to easily switch between the game"
This is the reddit link:
🧠 [FIX] EA FC 25/26 Massive FPS Drop After Halftime / Cutscenes (PC) — Finally Solved It 🎯 : r/EASportsFC
As crazy as its sound was the only think that worked for me, that's how bad this game is optimized in pc, and this problem hasn't been issued by EA since the beginning of the game it just gets worse from every patch. Hope this help many people dealing with this issue.