Also, Release tests updates to the current branch and just moved from Threshold 2 to Redstone 1
Slow and Fast are at the same level as Release right now, but that soon will change.
When Redstone 2 builds start testing they will go to Fast, then a few kinks get worked out and it drops into Slow.
So Release is the most stable/conservative. Fast is the most experimental/risky. If you want to test new features coming after the Anniversary Update with not too much risk, Slow is the best bet...