Tom's Hardware checked out a bunch of 2.5" notebook HDDs:
The small physical dimensions of 2.5" platters also have an impact on performance, as the highest absolute linear speed under the heads is lower with smaller discs. In addition to this general disadvantage, most 2.5" do not run at the 7,200 RPM speed that is common in the 3.5" space; they spin at only 5,400 or even 4,200 RPM.
For many years most drives were the slower 4,200 RPM speed. Yet there is a wind of change in the notebook hard drive space, because seven of the eight new drives we received for review spin at 5,400 RPM with only a single one at 4,200 RPM.