When I play a game with my brother on the same server, the connection to the server struggles. However, when my brother is on a separate server, the connection is fine. Keep in mind that on either condition, we are both playing the same game and should theoretically use the same connection limitations.
This quirk has been happening for a few years at least. It happens, as far as I can tell, 100% of the time on any game on Roblox. Network intensive games seem more prone to the effects of course.
In this image, my brother is sharing the same server and same household network as I. The network problems make the game nearly unplayable.
During this screenshot, my brother is playing on a separate server, but we are both playing Phantom Forces. The network behaves as intended.
This was taken only fifteen minutes after the first screenshot. I never left the server, and after he joined a different server my connection stayed strong.
I am not certain, but the network seems to degrade the longer we play together. We have had this issue through multiple routers and modems, including switching internet plans and the type of modem altogether.
I reset my computer recently and re-installed Roblox, the issues persist.
Additional screenshots as requested in DM: