The Roku devices (when wired/using ethernet) are capable of Fast Ethernet. (max transmission rates 100 mbps).
If you connect wirelessly/wifi to your Ultra and other ethernet capable Roku devices, you should see the higher speeds of your new internet reflected. (Settings/Network/Check Connection)
As a sidenote, assuming you meant Premiere+ (4630), rather than Premium+.
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660x, Premiere+ 3921, Express 4k+ 3941