Apple-provided software to measure or estimate the CPU load associated with Core Storage
iotop would be the first place to go if Activity Monitor wasn't getting you an answer. It uses dtrace instead of the activitymonitord and has far superior probing skills without the "needing to learn DTrace" part.

Instruments would be my second choice but it has so many presets, it might be some probing until someone answers with an easy 1, 2, 3 to reveal the specific workload of FileVault 2.