Quote Originally Posted by dariathalon View Post
If I think that an early MM monster is likely to slow the game down too much (too many hp for example), I find it's pretty easy to modify them to line up to the new math. Taking a couple of minutes before the combat as the DM to do this change before the combat can be easily made up in the time it takes to run the encounter. As the DM you can do this in advance of the session for most combats, but if the players go off plan, it is fast enough you could do it while people take a bathroom or snack break.

I'm not really a fan of your rule, but I will admit it does address the issues that you set out to deal with. My problems with it are the same as others have mentioned. You've added another choice for players and DM both for every attack. At my table, pausing to make choices slow the game down rather than speed it up. You're also reducing the randomness that makes the game fun. If I really need an attack to hit, it isn't hard to spend enough hp to make it almost automatic. While that has a cost, as you go up in levels it gets less and less painful to pay, and you have full control over paying it.
In general, if you find that it adds an extra choice to the table and find that is a bad thing because it slows your combat down you just need to have a standard use of the ability, both as a DM and as a player. My suggestion is that you always go and trade the maximum possible hp for maximum possible extra damage. That way you keep the randomness, you have a standard choice (so you don't slow down the game) and you make combat fast because everyone loses more hp and deals more damage.