Other things that slow down the game:
- Marks
- Immediate Actions that require some thought on when to use them
- Readied Actions + Delays
- Not knowing the defenses you're trying to hit.
- Powers that cause frequent changes in to-hit or damage
- Powers that cause effects that are resolved later
Basically anything that happens outside of your turn and isn't trivial: e.g. Dimensional Vortex (swordmage 3): it's an immediate action ranged attack. This requires serious interruption because it's a die-roll, and furthermore, it requires some thought (1/enc but usable commonly more often than that), and, finally, it's ranged, so it in turn can provoke OA's.
Powers that grant temporary bonuses and/or penalties really up the amount of administration needed. The constantly fluxing penalties on various enemies which further differs per target due to marks and/or cover/concealment coupled with the everchanging bonuses (ok, now
you have
this bonus but only vs. him - oh, no on all attacks or whatever) means that many attacks involve a time-consuming "did you remember this penalty/bonus?" phase.
In small parties, there will be a small selection of these kind of bonuses meaning that the same thing comes up all the time. In large parties, you'll see multiple leaders+controllers, and things get chaotic with different types of bonuses that may or may not stack and that have different limitations - not to mention that there are obviously more effects in-flight at any one time.
I'm not sure that'll fly in your group, but you
could try banning marks (and that means essentially no defenders), and frowning on too many leaders. Add in circular initiative, and you should have quite a bit fewer interruptions each round, not to mention a less grindy party composition.