I think it's slower because you have to get into the car you want to upgrade, then go to the items select button, then select items (instead of paint),then find the item, then select it, then select install.
Plus if you buy some extra items (for whatever reason) you tend to forget about them and buy the same parts in the tuning shop.
I buy a half dozen sets of sports tires sometimes, but not much else. Also, if I know I'm about to add parts to a few cars I'll buy the parts all at once.