Conversation after my open mic last night with a guitarist in the audience:
Me: (confused look) I'm sorry...?
Guitarist: (examines looper) Well, you're using the DL-4, right? Yeah, I've got the same pedal. But usually it slows down.
Me: (still confused) You're using the sample function?
Me: Well, that doesn't slow down. There's no feedback function on the Line 6.
Guitarist: Yeah, but when I play with a drummer it starts out okay, and then the loop slows down.
Me: Then your drummer's speeding up. Is he playing with a metronome?
Guitarist: No, man, he's a jazz drummer, he's rock solid.
Me: Well, something's off. I mean... Look, do you practice on your own with it?
Me: I don't know what to tell you. The loop needs to be very precise.
Guitarist: Right. I guess so. I love the sound of the pedal, but it keeps slowing down...
If you decide to do what I do, and by all means you should because it's a great way to practice and build a sense of rhythm, learn this well: the loop does not change. It does not slow down. It does not speed up. That is the whole point of a loop. And because it is mindless, the loop is god. It takes time to get used to playing over samples, because they won't react to you the way a regular band does, but it will teach you to be a better listener, and I've noticed that I've gotten much better at maintaining a tempo since I started.
You have to get it right the first time, and then change to fit the loop if necessary. Because the loop will not change to fit you.
I'm willing to bet that drummers and many bassists especially have trouble with this. As the rhythm section of a traditional band, we're used to setting the dynamic--which is great, that's our job in that role. When you play with a loop, your role must, to some extent, become more of a soloist/accompanist. No matter how solid you think you are, the loop does not slow down.