If the point is to reduce the amount of time one player spends during his turn, I think the better solution is to coach the player into resolving his actions faster. I'll refrain from pontificating on that subject unless it is indeed the point.
If the point is to provide multiple-attack characters additional tactical options, various of the suggestions so far seem adequate. I like Igfig's the best.
One option our group uses when pressed for time is to roll damage once for the round and then just make the proper number of attack rolls. Simply apply the damage per hit. next round we roll damage again so its not as static as using average damage and sometimes you have a good round and sometimes you dont. If I crit I often just double the result of the dice I rolled instead of rolling another set again.
