For it to run a whole lot worse. I can run RTW on medium setting with large units without any hassle, but MTW2 is slow and choppy by comparison, with less detailed units. Certain areas of the campaign map run smoother though, namely The areas covered by the Byzantines, the Muslim Factions and Russia. Italy is the slowest region.
My laptop which is subpar for gaming runs both games on medium settings, though I do not do the actual manu battles for campaign mode, and I don't seem to have much issue with it. I don't really use any mods except for the above mentioned unit replacer for the Spartan Hoplites but for Medieval 2 I use Retrofit on the occassion I play the stock campaign, otherwise I mainly play Third Age.