Network Connectivity Issues When Playing Intensive Games

Hello, my friend has been having issues with his internet when trying to play games, specifically graphically intensive ones. Smaller games like Bloons TD have worked fine, but games like Marvel Rivals and Path of Exile 2 have had serious issues with lag and connectivity that prevent him from playing. Specs should not be the issue (CPU: Intel Core i5-10400 | GPU: RTX 3060), we have been trying various fixes for days, including checking and replacing cable connections, attempting both wifi and ethernet, etc, the issue persists. Did not used to have this problem.

In theory it's a PC issue since low-intensity games work right? High-intensity games prevent game connection, cause audio and stream cut-outs on Discord, etc. but everything seems to work fine on "weaker" games. We're just lost on what it could be or how to fix it.