I've only run a Warhammer FRPG 3rd game PbP so far, and I must say that combat annoys me the most. It just takes forever, even with the (comparably pretty speedy) WFRPG system. It helps if your players know and make use of all their options, and if you add drama to the conflict in some other way (a chase, working against the clock, hostile locations). I also tended to give my players a lot of leeway here: once the biggest challenge was overcome, I would either just roll out the last few rounds of combat or say "... and you finish of the last few baddies" or have them flee.
On pregens: if the world you run in is commonly accessed or easily imaginable, then I don't think pregens are necessary, they just add to your work. Give the players parameters ("don't build a pure fighter, we're going to do little combat", or "magic users are killed on sight") and let them run with it. However, if you are using a less well-known setting or one you developed yourself that is completely off-kilter (like, say, a post apocalyptic fantasy setting, or a cyberpunk port to fantasy), I'd pre-make all characters, and if the players are interested in continuing after the first adventure, allow them to make changes or swap out characters. This will save you time; you don't have to provide mechanics for all options available, heck, you don't even have to describe all options.