As documented above, Roon/Tidal poor behavior was cured when Core was tethered to a T-Mobile cellphone. And all other streaming clients on my network are fine on ATT network. ATT is known to throttle Tidal on their cellular network. Those 3 clues suggest ATT is throttling Tidal on my fiber network. So I tried VPN to hide my Tidal requests from ATT, and it worked.
Using VPN client solved this problem = ATT is throttling Tidal.
The Tidal desktop client did not have the problem, and Tidal desktop app takes forever to start. Both clues suggest Tidal desktop app is using built in VPN client.
Hope this information helps others who have poor Tidal performance in Roon.
I am using TunnelBear for VPN. It seems to work well, and costs $60/yr for 5 clients unlimited data.