It's because you're using Adaptive Resolution, where AE will attempt to let you scrub through quickly, by reducing the resolution it shows in the viewer. Higher powered computers will be able to show higher resolution in the viewer more quickly.
There's an explanation in Adobe's help here:
There are also many things you can do to adjust the quality of your preview, depending upon what you're working on.. For example, if you're checking the movement of something, maybe you don't need as high resolution an image but full frame rate... Whereas if you're checking the pixel-perfect quality of a matte, maybe you don't need as high frame rate during preview.
Also bear in mind that there are different render engines in AE now - if you're using the 3D raytrace renderer for 3D work, previews will be very slow unless using a super-fast machine.