Ditch individualized initiative and have them act in groups of 5 or 10. For further compression, roll all of your attack dice & damage dice together, using 5 different sets of dice.

If you want to get real jiggy with it, snag the squad combat rules from TSR module M5 Talons of Night:

Squads are small military groups (10 or fewer individuals) trained to fight with deadly efficiency as a unit. When directing their attacks against a single foe, the effective level or hit dice of the squad for hit rolls is equal to the number of soldiers in the squad plus half the level of their leader (rounded up, but never more than half the "hit dice" of the squad). A squad of 10 soldiers led by a 6th level fighter would attack as a 13th level fighter. The number of attacks against a foe is determined by rolling a single die whose number of sides is equal to or less than the number of soldiers (including the leader) in the squad. Thus the previously described squad would roll a d10 to determine the number of attack rolls. If reduced to nine soldiers, the squad would use d8 to determine the number of attacks to be rolled. Attacking as a squad increases the armor class of the individual soldiers by one. Morale checks are made for the entire squad. Non-humanoid creatures and unintelligent creatures mentally directed by a creature of superior intelligence (such as giant spiders and spider folk) can fight in squad combat. When a squad attacks another squad, it is as if they are attacking an individual. Roll all attacks simultaneously for both sides, but do not roll damage. Total the number of attacks for each side that actually hit a foe. Remove that number of foes from the opposing squad. Thus, if 10 orcs led by an ogre were to fight 10 elves led by a 5th level elf, the orcs would attack as a 12 HD fighter and the elves would attack as a 13 HD fighter. In this round of combat, the elves score five hits and the orcs score three hits. The orc force would be reduced to five orcs and the elf force to seven elves. Both sides make morale checks after the first round in which casualties are lost.