Unplug the Mackie mixer. Wait one minute. Plug it back in and fire up the Mac. If it still bogs down then try a brand new firewire cable. This sounds like a bad connection or a cable that is about to crap out. Remember: do not wrap your firewire or usb cables with tight bends or stretched around corners of equipment. The stranded wire in those cables is very tiny on the order of 24 or 28 gauge. I know they come from the store wrapped tight but it's the repeated bending that breaks the wire.

And if none of that works, it could be a corrupted driver. Then you would merely try reinstalling the iOnyx ASIO but it seems to me it works with Mac's Core Audio?