Multiple editions had combat would easily leave 15-30 minutes between turns at high levels. 15-20 if everyone paid attention, 30 if you needed to reexplain things again and again when players lost interest while they had such a long down time. It was a nasty spiral where some players getting distracted meant recaps of the last round which bored the other players and wasted their time so they were more likely to do something on their phone, etc.
Really, combat is the #1 place where I see people disengaged from the game.
Yep, I saw a lot of that back in the day. Not just in combat, out of combat one or two players would usually be more assertive and everyone else would just tune out. It was annoying. I'd always try to keep everyone engaged by actively asking what each player was doing when they seemed disengaged. On occasion, I'd have a player just up and say they were waiting for the action start or otherwise temporizing in a way that made it clear they were disengaged because they weren't interested. Most of the time, they'll do something. I've ended up running some very disjointed scenes that way, with some players interacting in one scene while others get into mischief nearby, for instance.
It's less of a problem the more cooperative the game's resolution systems have been, and, ironically, more of problem the less of a disparity there is among sub-systems & how long they take to resolve. When diplomancy can wrap an interaction scene in one roll, there's less disengagement from the wake-me-when-the-fight-starts crowd, when the wizard's turn takes 15 minutes, but everyone else is done in 1, that's a 20-minute cycle, but when everyone takes 5 minutes, that's 30+ minutes to cycle.
And there really is a 'cliff' some tables fall over. When the cycle gets long enough, they disengage, lose the thread of what's happening, and then each turn adds on having the situation re-explained to each player and the cycle gets that much longer. It's especially brutal running TotM, as you have to re-describe the whole lay of the battle and how it's changed not since the last turn, but since that player's last turn.
Also with a certain amount of irony, the more a PC has off-turn actions (notorious for 'slowing the game down') that the player likes, the longer that player can stay engaged in the encounter between his character's turns.