I have a client that runs a lot of WF scheduled scripts every 30 minutes. Normal CSV and mass updates. They have SuiteCloud Plus with 10 queues. I wrote an M/R that is using 4 but it seems like everything is taking a lot of time to process while this is running.

this behavior may depend on what your MR script does. Even with multiple processors, I think EVERYTHING running on your account is constrained by the shared database. I think it's quite possible that contention at the database level can cause problems even with multiple queues.