I will say, however, that I am blessed with players who are not on the phones and are always ready when their turn comes around.
It's something that's been discussed here before (as I think you mentioned in the OP), and I've been trying to nail down exactly what the root cause is over the past year or so.
With the understanding that the common denominator in my observations is, of course, me, I don't think I'm the root cause. My turns are routinely speedier than the other players, even when I interject flavor text and extra narrations into my actions. And the games where I DM are generally faster paced than some of my other DMs, although not to the speed I would aspire to.
I've noticed three major aspects that really slow up combat, in increasing order of "fixability".
1) Slow speech patterns. Some....people...just....talk....more....slowly. I have 4 players (across multiple groups) who simply don't talk fast, both in and out-of-game. Nothing like an impairment or a disability; they simply draw out their sentences as part of their normal speech pattern. Telling people to talk faster seems both rude and unlikely to work, so this is something that just has to be accepted.
2) Focus on stunts/negotiations/non-standard actions. I have 5 players (across multiple groups) who like to try something weird at least once or twice a battle. Some weird stunt that involves multiple grapple checks and positioning, trying to talk to the enemies during combat, casting a new spell they've never used before and aren't quite sure of what the implications are, etc., etc. Combine this with a newer DM (2 of my groups have first-time DMs) and trying to do this on online battle maps, and this is the source of a lot of 10+ minute turns.
3) General hesitancy. There are way too many players in my groups (at least half) who express their turns as questions, not statements. "I would like to move here and attack......right?" instead of "I move here and use my action to make 2 attacks, here's my rolls" is way too common an occurrence. This seems like it should get better as people get more familiar with the system, but some people are simply not "rules people". Excellent at in-character dialogue and character inhabitation, but the rules minutiae seems to be a permanent challenge.