...and I would argue that how much time it takes for each player to get their turn is a gameplay issue, when players aren't actually doing anything but waiting for their turn. Being the only player doesn't erase it, it just spreads it out over however many NPCs are involved, both in the player's party, and the opposing faction's party. There have already been threads about slow combat on these forums. That will be compounded with the addition of more characters in the player's party.
I agree that BG3 combat can currently feel slow, but this problem shouldn't be exacerbated with an (optional) increased party size:
1.) If Larian doesn't change encounters (many people are arguing for this - I think Larian should change exp gain instead) then a 6-person party will have a higher ratio of players-to-enemies than a 4-person party. I.e., a single player will spend a higher % of their time actually playing the game vs waiting. And in absolute terms, a 6-player combat will then proceed
faster.
2.) If Larian does change encounters for a 6-person party, ideally they'd do it smart (which, hey, might be a big assumption) and modifiy enemies so we face a similar amount of stronger enemies, rather than more of the same weak enemies. Thus time spent between turns and in combat would remain the same.
Individual play time will be negatively affected in multiplayer, but only if you decide to play with 5 friends. Which is an additional option not currently allowed, and I want to stress,
is only an option. You can still choose always choose to play with 4 players and 4 PCs for the default experience.