No, using um IS slowing down and taking the time to form full sentences. Or at least it can be and often is.
Maybe I'm creating a false dichotomy.
The act of just unleashing your words without planning them first is not a lazy way to talk in the sense that it takes the least quantity of effort. Obviously that requires more work because you are trying to keep up with high velocity communication. When you've set a pace that's too fast for yourself you use filler words. You can't possibly keep up, so you need to stall for time without permitting a pause.
Some people genuinely are worried a pause will lead to the other person jumping in. But I think most people are terrified of silence because we aren't used to it. It's not how we are trained to speak.
When you take presentation classes however this way of speaking is completely inappropriate. Filler words are lazy in the sense that it's what people are used to so they try to fall back on it. The amount of effort to force yourself to control your pace of speech and construct good sentences and refrain from using a filler word is I think in the balance harder to do, at least until it becomes a habit, then of course it takes less effort.
edit: So I guess I see filler words not as a means to form full sentences, but rather to let your mind move further ahead while your mouth stalls for time. What you should be doing is form a full sentence first, and then as you talk speak as fast (or as slow) as you need to let your mind stay on top of things. Sprint to the edge of your prepared notes and your brain stops preparing and just says things as they come, and the result is lazy speech in that it's thoughtless.
