I've had the same issue. The giants and also seem to congregate around the base cluster and look out over the chasm to the battles... Than occasionally they wonder off to get stuck. I have discovered if you lure the other monsters or hero to the stuck creature, they will become unstuck and surge to the enemy base. Once this happens and they are killed the slowdown is reduced, at least on my machine.
[Suggestion} Add a function to kill friendly creature and the ability to attack and destroy friendly tower and wall segments.
CPU INTEL|C2Q Q9550 2.83 Over clocked to 3.43GHz
|ABIT IN9 32X-MAX LGA, Bios v1.4 | Zalman CNPS9300 AT CPU Fan
CORSAIR Dominator 8GB (4 x 2GB) DDR2 1066 (PC2 8500) w/ Cooler
EVGA GeForce 8800GTX KO HDCP Video Card, Driver V178.13 x64
2x Western Digital Raptor 74GB 10,000 RPM
Plextor PX-7555A DVD Burner | XP Pro x64 Edition
2 Dell 2407WFP 24" LCD Monitors