Queue Processors make it unnecessary to create custom queue classes that extend System-Queue.

The only class you need is the applies-to class of the persisted step page at the time Queue-For-Processing is called.  Typically the persisted step page would be a case.

When the step page is restored to the Clipboard later, an Activity is run in the context of that restored  "bootstrap" step page.  You are asking whether it is best practice to persist an instance of a "custom queue class" to contain the "bootstrap" data

You do not need a class that inherits from System-Queue-.  Queue Processors use the Access Group associated to the ASYNCPROCESSOR Requestor Type as shown on the rule form. Certain properties in System-Queue- would not be used most likely.

You could, instead, persist an instance of Data- class, then use that as your step page. It would be your responsibility to remove those instances when no longer needed.

My recommendation is to consult Pega experts on this as you asking about database-level performance which can vary depending on the database vendor.

Concerns:  date-wise partitioning, indexing, query parallelism, defragmentation...

Having a different class, hence table, per Queue Processor makes perfect sense.

I am not the right person to ask. Wish I was :- )