I agree with the lag.I get the same issues.
Especially when the round is over, when the camera tries to pan around the arena.
It stutters and jerks and freezes.
And I have a decent system too (2.5 core 2 duo, 8gb ram, 8600GT 256mb, WD Caviar Black HDD w/ 32mb cache).
However...
I have to disagree about the Rook! 
Other demigods could move just as fast if they invested in the movement items too!
I think it's just that the Rook focuses on those items due to his initial slower speed.
I've played against an UB who could outrun my fully upgraded Rook.
Although, I would hate to go up against an UB that was able to purchase the All Father Ring, the Belt of Giants, the Mageslayer, and various other items. Ouch!
As the Rook, by the second half of the game I usually have those from saving and waiting then I can demolish other demigods. But without it, I'm slow and fairly week. Effective against buildings and minions, but not so much against demigods.