Before I stopped playing WoW last time, I moved the game off the SSD and back onto my old-timey HDD. It definitely loaded faster on the SSD, but performance otherwise seemed to be about the same on the hard disk.
Tonight I logged in after many months away, the game fully patched and shiny. I then experienced some of the worst non-network related lag I’ve ever seen in the game. The framerate went from a high of 60 to as low as 18. Your framerate should never dip below legal gambling age. It was unplayable bad, which prompted me to stop playing. It’s possible the game was caching files after being moved back to the hard disk, but whatever it was, it did not leave me with a burning urge to try again.
But I will, for science!