Weird performance behaviour in 7.0

[Bruce Albrecht]

This is my first Q6600 based system, and my first 7.0 system, and my
first AMD-64 system and my first ZFS base system, and I'm seeing
something really strange.  Every now and then, processes run really
slowly, like at 1/600th (or worse) than it ought to, but it racks up
the CPU time as though it's running full tilt.

For example, I have this little test program that just does a tight
loop, and most times, it takes about 3 seconds to complete, but right
now, it's taking about 2000 seconds to complete. Right now, it's
consistently running slow, but sometimes it will run slow, but I can
terminate it and start another one which will run at normal speed.

Currently sysctl reports:
dev.cpu.0.freq: 2400
dev.cpu.0.temperature: 55
dev.cpu.1.temperature: 57
dev.cpu.2.temperature: 51
dev.cpu.3.temperature: 54

What might be causing this?  What information is pertinent for
inclusion in a problem report?