Hey Bruce LA of 2.5 but CPU under 10% suggests that you're hitting IO limits. What's the %sys or %wa in top? I'm not familiar with bitcoin, but you said you had 4.5GB of blockchain. If this means you've got 4.5GB of data on disk, and bitcoin is doing a lot of random accesses to it, it will definitely cause this kind of loading. If you had loads of spare ram you could try moving your blockchain to a ramdisk during the verify, then moving it back once you're done. Alternatively, if you had lots of spare ram, everything might end up fitting nicely in the fs cache, which would also improve random access. Otherwise you're looking at improving your disk IO system, which probably means an SSD.
So just for a test I'm running it again now. I checked and there isn't even the option of CPU mining any more.
My load average has jumped to 2.5, it's normally around 0.1
CPU and memory used by bitcoin seem quite reasonable (both under 10%)
At the moment other programs are behaving but I have bursts where everything gets painfully slow. When I type, it takes five or ten seconds for the words to appear. Firefox keeps going grey. This ONLY happens while bitcoin is running, if I stop it everything goes back to normal.