But the other two tvs use Netflix and not seeing this sort of bandwidth degradation
ROKU must be taking priority somehow.
Ultimately thee is only X bandwidth to distribute on a given internet connection. For every device added there will be even degradation (sucking up a portion) unless some are prioritized. Box 1 may have priority and then Box 2 will get any leftover bandwidth so that Box 1 has access to full speed and the rest get it’s unused ‘cycles’ so to speak.
Roku should not simply degrade bandwidth simply being attached and no viewing occurring. If it is, something is very wrong.