This seems to be helpful. (Not that defragmenting isn't a good practice) but particularly if you're playing RNG Roulette (Microgaming, Playtech, etc.)
If you play the game in your browser, not a downloaded exe, the game should not have access to your disk, more than to place cookies. I never use downloaded casino exe programs.
So start defragmenting after a session will not do anything better is it not happens to be good to do it anyhow.