Yuuki,
Yes, I would agree that, if the speed and killing rate do pose an issue, then this will all the more apparent in MP. One can always mod the SP. Having experienced major slow downs and stuttering (to 5fps) during a modded 2 vs 2 demo battle involving large sized armies, I have my doubts as the full potential of MP anyway.... at least for greater than 1 vs 1s.

My PC is:
ATI X800 Pro
Athlon XP 3200+
1 GB RAM

Louis,
I'd say a mixture of both - Endless reinforcements obviously lead to increased time, but so can marching across an entire map to meet an AI army halfway up a hill, or chasing routed troops... I found myself constantly using the timer at full-speed, which in itself tends to detract from the atmosphere and epic feeling of a battle.