I just spent the last few months updating my Unraid system with an updated Supermicro case with 12GB SAS expanders, 12 more drive bays, bought new 2x 12gb HBAs, updated the CPU to lower power, lower speed (but with 4 more cores)... Did all the testing, using old crappy drives, assuming they were the reason parity was slow. Went ahead and moved over the real drives... the end result was 1/3 the speed of parity check, from ~24 hours to 3+ days... Today I figured out that the parity check is single threaded. Just bought some overkill CPUs that are twice the clockrate and 70% more power usage. I'll try to reply once I get the CPUs replaced.
In short, it would be nice if the parity check/build was multithreaded.
