As I understand it, on systems with less RAM (Laptops & Netbooks,etc) installed it may cause the OS to run slower, as it'd take more cycles to remove the ad from the page being loaded, i.e it takes time to run the ad block from pre-loading the page until the page is actually displayed. However, as most desktops have a relatively large ammount of RAM installed, then the "time lapse" on the ad block is not very noticable - hence the computer seems to run faster on a desktop.
The above is only MHO, and stands to be shown as incorrect by other users. :)