To expand on this a little bit, I think much of the magic here happens in the narration. In other words, when you are narrating the results of a player's move (successful or not), you can flow more or less seamlessly into the monster's action/reaction. So instead of "You Hack & Slash the mummy for 5 damage. What do yo do next?" put more detail into it and end with some action on the monster's part that forces the players to react. So maybe, "You swing laterally, your sword tearing deeply into the dusty, tattered wrappings on the beast's torso. It staggers back from the weighty blow, but recovers quickly. Worse, it emits a dry, raspy chortle, and you see shiny black scarabs pouring from the wound you've just opened. The scuttling swarm of insects is lightning quick and heading straight for you. What do you do?"
The first method doesn't allow the monster to act at all, while the second makes its action part of the result of the player's move.
Note also that this is a basic (by which I mean not "hard") GM move (in this case, reveal an unwelcome truth). You aren't saying, "scarabs pour out and start eating your liver, take X damage." But then again, were the player to fail on his hack & slash move, that might be appropriate; it is the "hard move" equivalent of trade damage as established in that you're narrating both the situation (ZOMG, scarabs!) and its outcome (they begin feasting on your liver!) at once. But just by putting the scarabs in play at all, you are effectively allowing the mummy to "act" based on the player's move in a way that is seamless and fits in with the fiction as it unfolds. It also fundamentally changes the fictional situation, which will help minimize the risk of dog-piling - because I can virtually guarantee you that the answer to "What do you do?" isn't going to be answered with, "I ignore the scarabs and hack at the mummy again!" If anything, it may be more along the lines of, "I piss myself and run!"
And on the topic of moves taking time, consider "set-up" moves. So for instance, if two players are both trying to hack & slash the same foe but there isn't sufficient room for them to spread out and not be in each others' way (or perhaps if the Ranger is shooting at the monster with a ranged attack while someone is in melee with it), make them set-up their attack. "OK, Takhsin, you want to attack the mummy. But Ragnar is in there like a whirling dervish with his axes, hacking away at the towering thing. In the narrow confines of the darkened tomb corridor, you'll need to time your attack pretty carefully to avoid hitting him - go ahead and defy danger with INT." Then use the result of that defy danger to direct what happens next. If the player aces the roll, then sure, let him get in an attack right away. If he partials it, offer a worse outcome (all you can manage to get in is a hasty jab, but it's pretty awkward and weak, -1D4 damage), a hard bargain (you get in a solid hit, but take one of Ragnar's axes on the backswing for 1D6 damage), or an ugly choice (you can either forego your attack, bide your time, and take +1 forward, or you can wade in now and get in a solid hit, but interrupt Ragnar's timing in the process and deprive him of his next attack). And if he fails? the world is your oyster.
As always, the ongoing fiction should dictate what the PCs' options are, and you should frequently be giving them a situation that requires them to react or face serious consequences.
Does this help?