I don’t know for certain but I don’t think running your own router will decrease internet speed by much if at all, I’ve never done any testing to support wether it does or not, but I went from using Xfinitys modem/router getting 800up/25down to using a unifi udm pro, and I can say I consistently get the same speeds, I just get more control of everything that happens on my network. In my opinion switching to any router of your own would be best no matter what.

Also as a side note if he isn’t playing on Ethernet, I greatly doubt that the internet speed over WiFi will be affected at all if internet speed does drop at all from the router swap!