To add to the injury swapoff doesn't read from disk sequentially, but in some "random" order, so if you're using a hard-disk it's a huge pain, although even SSDs would benefit from this.
When I was last messing with this ~10 years ago, even with SSD swapoff was just insanely slow. Even relatively small single digit GB swap partitions would take many minutes to drain. I think it was loading one page at a time from swap or something.
