- The monster's extra actions means the DMs turn takes longer, the triggered attack both takes time to administer and adds to the player's thinking time as they have to include the risk of triggering it in their own (and sometimes other people's) decision making. Is it worth my fighter triggering the reaction if that will free up my rogue to do what they want without risking it?
- And the ongoing damage - save ends? So that's a save or 3 most turns by players. Something more to remember, something more to do, something more which might trigger feats or powers and something more for the leader to decide if they need to fix
I think a significant part of the reason fights took so long is the high number of ongoing effects and the range of rules for when they end - end of my turn, end of your turn, end of next turn or save at the end of the turn.
All of that makes for fights which are interesting. If you like tactical boardgames, this is the edition for you - it's quite good for that.
But I think you could halve HP even on the later monsters and still have enough of that (with character HP reduced too).