Why you should have a slow, spinny loop
(This was originally from a forum posting, but I thought I'd put it here as well.) It's extremely helpful to have a slow, spinny loop, for four reasons. First, many players have difficulty with slow, spinny loops, and if you don't have one, then you are handicapped in the match. Second, it gives you more variation, which makes your other loops more effective. Third, against a very low, heavy push, it's much easier to go for a slow, spinny loop then to try to power it all the time. And fourth, if you are missing your faster loops, it's good to have a slower and steadier loop to fall back on.
There is less slow looping at the highest levels, but that's because at that point they can pretty much rip anything they see. However, even there you'll see some slow loops as variations, depending on the circumstances. But anywhere below the world-class level a slow loop is one of the more underused shots.
How do you do a slow, spinny loop? One key is to let the ball drop more than usual, especially against backspin. A common mistake when slow looping is to slow the swing down. Instead, use normal power, but graze the ball so finely that most of the power converts to topspin. Then get ready for your follow-up - and note that the very slowness of your shot gives you time to prepare for the next shot. This is why when you step around your backhand corner to forehand loop many players either loop a winner or a slow loop. If you loop medium, then unless you have fast footwork you might not have time to react to the likely block to your wide forehand.
Training session on forehand