Oh man, Good thing Halo 5 has dedicated servers!... . . :P
OP: likely scenario, the servers used for H5 dedicated hosts, have poor routing for your IP block / location, and the DSL service you tried, while slower overall, has better routing to the dedicated hosts, resulting in better performance, I did try and explain this point in another thread but many predictably ignored it.
Possible solutions: reboot router entirely, if you're on a dynamic IP it *might* drop you on to another IP block thats has better linking and less hop congestion, if you're on a static IP, call comcast and ask them if they could change it for you, specifically state your reason and they should be able to help.
If they were using P2P network model for multiplayer, your solution would simply be "Leave the round and join another".
