Internet speed slows to <5mbs only when playing online games

My  internet speed is over 800mbs/s, my computer is connected by ethernet cable. when I'm not playing online games my download speeds are actually faster than what i pay for my plan at around 850mbs/s. when i log into a game lets say fifa 23 things will be ok for a few games but then it'll start to lag and eventually kick me out of the game. If i run a speed test while i'm still in the games menu my download speeds will be between 1 - 5 mbs/s. I've tried connecting through wifi but that doesn't help. As soon as i shut down the current game and run a speed test it jumps right back to 800-850mbs/s. I play fifa 23 through steam but have the same problem when playing COD MW Warzone which isn't through steam. I've called xfinity support who actually increased my internet speed from 400 to 800 for free which required me to pay for the newer router ($15/month). oh and while i'm playing an online game and getting poor internet speeds all any other device connected to the network gets the same bad internet speeds. 

I'm hoping someone is able to help me correct this issue as it is very frustrating. I'm gaming skills are bad enough, I don't need my internet speeds to help my opponents get a kill or win the match.