These tactical battles take forever compared to what we get in Civ 6. That's fine in single player games, but it's really boring to wait around while some other player is engaged in tactical battles somewhere. Also, you get weird map interactions where you can't attack a player's units because that player is already fighting something else, so you have to wait around. It's just awkward.I'm not really sure what "it slows down the game" even means. Like, it takes more minutes to complete a game? Who cares, as long as you enjoy the game you're playing.
But, if they're only allowing auto-resolve in multiplayer, then none of this matters. Instead, the complaint might be that auto-resolve removes some control from the player and doesn't allow a player to test their combat skills against another player's skills, which is kind of lame.