Persistent lag on Roblox over Xfinity

I'm having a persistent connectivity issue affecting Roblox and Discord voice. I've already replaced my router and factory-reset my PC. The same PC works normally when connected through USB cellular tethering, but the problem returns immediately on Xfinity Ethernet. I'd like help investigating a possible network routing or real-time traffic issue. I have additional troubleshooting results I can provide if needed.