Is it possible to assign different CPU priorities...

[neo0.]

To the mat preview render and the "final" render? Reason why is that with all of your CPU rendering that material preview, it should converge pretty fast, but with the final render, you obviously have to commit much more time to the render, thus using up your CPU for a much greater time frame.

[Whaat]

To the mat preview render and the "final" render? Reason why is that with all of your CPU rendering that material preview, it should converge pretty fast, but with the final render, you obviously have to commit much more time to the render, thus using up your CPU for a much greater time frame.

It is possible. I made the decision to have previews render at 'low priority' so that it is easy to launch several previews at the same time.