How can I monitor MacBook USB bandwidth?
It’s highly unlikely USB can slow down your mac since it has a minimum of 2x Thunderbolt 3 ports and likely 4 of them. The bandwidth there is way more than USB could saturate. It’s much more likely you need to look at the OS and apps running and rule out interference or USB cable / hub issues with some troubleshooting to note exactly what’s connected and do some timed measurements of whatever you care about when the response is normal and then when the response is slow.
A quick check of System Information should show you the USB speeds for each device and as long as they are the same when it’s fast and when it’s slow, you can know to look for things that change - and not worry USB is somehow intermittently slowing down.