When you say PbP do you literally mean play-by-post(al mail, i.e. snail mail)? Or are you using that as a generic term for what is actually play-by-e-mail? And, how are you running the combats? Do you handle like you would at a gaming table in person, where you send a post after every action by every combtant? If so, perhaps you would want to consider how you RUN the combat, rather than the rules of how it takes place.
I've been in 3 or 5 PBeM games, and we've gotten a good system down for handling combat. As players, we take time to set up some general combat strategies ahead of time and let the GM know what those are. We also make it clear to everyone how our characters generally act, and what their goals and priorities are in combat.
Then, when a combat occurs, we state which strategy we're following and what we want to accomplish, in general terms, for the first three rounds. The GM then runs those three combat rounds and reports back to us. (And those rounds are run intelligently. So if the fighter takes down his foe in the first round, he doesn't stand there for two more rounds awaiting new instructions.)
So with the combat being run in three round increments, it doesn't take that long for the players, and most of the work is on the GM - as it should be. This could speed things up for you as you don't have to send a letter or e-mail to each individual player after every single action.