"What kind of application blocks on disk IO but nothing else for extended amounts of time? I'm having a hard time seeing how installing an SSD and maxing out your cores are terribly related otherwise"
Virtual memory paging to/from disk. This is probably why the new MacBook Airs feel faster than the CPU+RAM specs suggest.
That'll improve your loadtimes, but I'm having a really hard time seeing that as the reason why most people aren't maxing out their CPUs all the time. Unless something has gone terribly wrong, you should never be hitting your disk that much.
During standard home computer operation, both the CPU and the disk are generally quite idle.
Virtual memory paging to/from disk. This is probably why the new MacBook Airs feel faster than the CPU+RAM specs suggest.