One of the common problems I see are players who stand too far from the table when looping against backspin. They'll stand well off the table, with their contact point in front of them. This means that to put their weight forward, they have to fall slightly backwards to compensate. It's either that or resorting to a rather soft loop, with mostly upward motion, and little speed.
Instead, learn to almost jam the table with your left leg (for righties) when looping backspin. This allows you to rotate properly into the shot, even with the same contact point as the players who back up, since you'll be contacting the ball to the side of your body instead of in front. But now, with body torque, you'll have tremendous power, and so able to loop at all speeds.
Having tremendous power does not mean you loop every ball with great speed. It means you effortlessly create power, and that power goes into both speed and spin. Go for about 50-50 on most shots, going for more speed against a weaker push or when you've really read it well. (Here's a Tip of the Week on creating "Easy Power.")
Standing closer to the table also allows you loop backspin a bit quicker. Some players even take them on the rise, but for speed loops, it's best to take them at the top of the bounce; for more consistent loops with a higher spin to speed ratio, take them perhaps slightly on the drop.
While this article was written about looping backspin, it all applies to looping against topspin as well, except the positioning is a bit more complex, depending more on the incoming ball. But against a softer topspin ball, you'd also want to stay close to the table and generally loop it around the top of the bounce, or even on the rise.
Here are three examples from the final of the recent Men's World Cup. These players don't push long a lot, so in the first game there were only three long pushes where the other player forehand looped, twice by Ma Long, once by Fan Zhedong. Watch how close they are to the table in all three cases.
So remember - when you want to jam a loop down your opponent's throat, jam the table!