big transaction slows down over time - but disk seems almost unused

[Ben <bench@xxxxxxxxxxxxxxx>]

I've got a long-running, update-heavy transaction that increasingly slows 
down the longer it runs. I would expect that behavior, if there was some 
temp file creation going on. But monitoring vmstat over the life of the 
transaction shows virtually zero disk activity. Instead, the system has 
its CPU pegged the whole time.

So.... why the slowdown? Is it a MVCC thing? A side effect of calling 
stored proceedures a couple hundred thousand times in a single 
transaction? Or am I just doing something wrong?