CPU usage depends on the hardware you are running! In 2.9 it is a squid system of some type, but all of that was repaced in v3.
In MT 2.9.x I tried both the Squid version and there IP test version. CPU load was massive on both while using a 2.6 ghz P4 CPU and 1G of RAM. Also some websites did not work and some flavors of Frontpage would not work. Switched that box to just DST-NATing port 80 to an outside Squid box. The Mikrotik CPU load dropped to a little of nothing. The Squid box running on a 2.2 ghz Athlon64 CPU and 1G of DDR rarely excedes 4 percent CPU load. THats handling 8mbps average(not sure how much is http), about 700 Internet accounts, no idea how many total PC's.
I just wander if doing all routing and caching was just asking to much of Mikrotik. I did clear the disk cache and set max size too zero for a while and it had no effect on CPU load.
Did you try the cache on 2.9.x and if so what were your experiences? I am just wandering if I should try it again in 3.x if its improved. Don't want to get custommers using Frontpage upset either. Its weird though. Most versions of Frontpage worked its just certain older ones did not.