There are better fillers to use than "um" and "uh", though, which don't reflect as badly on the speaker. As some random examples, you can connect things with "Now, [pause] ...", or "On the other hand," or "For that matter,", or many other connecting phrases. Those can be time-fillers too, but as long as they're not overused, they just sound like conversational language rather than making the speaker sound awkward or unpracticed.
Depending on the circumstance I've been working hard on just using silence - or when that would go on too long , simply saying, "I'm thinking that through" or something similar.
Anecdotally people seem more focused on what I'm saying when I do that vs the filler sounds I'm trying to stop saying.
