I found some latency problems with the MSR section of the Mackie a while ago. I reported it to CW, I'm hoping they are looking into it.
My advice would be to, double check the fault isn't at your end and after that report it to the backers, the more on here that use the unit the more chance we have of getting the niggles fixed.
PS. IMHO The ProChannel is a real pain on the Mackie, I hope they can find us a solution.