I don't believe in larger parties to speed up fight. Frankly to massively speed-up fights I often go solo just with my main buffed character (STR 18, CON 18, AC 20). The other party members just cast on him Bless, Armor of Faith, Longstrider and Jump and stay a way out of combat to easily maintain concentration. Being solo means enemies will not spread out to catch other (and much weaker) party members. I am something like fire or hot light bulb for a moths. But this strategy really works only if you have min-maxed you main character.