No, it's not slower than 1 or 2.
I look at the (real) clock a lot (I know it's not normal, but it's just the way it is
) and I see little to no deference.
Maybe it looks that way because Sims 3 is much heavier than Sims 1 and 2.
When I play sims 1 on my laptop, the loading times are far less than I remember them to be.