I'm no expert, but my guess is that the DirectX6 components that you need to install for ROR require a lot more work out of the processor. So AOE with DirectX5 may run faster than ROR with DirectX6.
I did reinstall AOE and ROR recently when I upgraded my computer. I tried to run ROR with DirectX5 just to see if you could. It is possible, but once you get more than 10 villies the game bogs down to about 1 game second = 5 real seconds or worse. So you definitely need DirectX6.
I know ES and Microsoft won't say it, but I think the computer requirements increased from AOE to ROR. The laughable P90 minimum should probably be increased, although I can't say how high. I've only played with my old P200MMx and my new Celeron 450. The P200 would sometimes turtle and lag, which is a big reason I upgraded.
Anyone else have theories on why ROR seems to play slower than AOE?
Mr Hanky