For example, a simulation program with a large data set that is accessed in a very random fashion would have almost no noticeable locality of reference in its data segment, so its working set would be quite large. On the other hand, an xv with many simultaneously opened JPEGs, all but one iconified, would have a very large data segment. But image transformations are all done on one single image, most of the memory occupied by xv is never touched. The same is true for an editor with many editor windows where only one window is being modified at a time. So for a configuration with 16 MB RAM, no swap is needed for a minimal configuration and more than 48 MB of swap are probably useless.
Mechanics are slow, electronics are fast.
Modern hard disks have many heads. Switching between heads of the same track is fast, since it is purely electronic. Switching between tracks is slow, since it involves moving real world matter. So if you have a disk with many heads and one with less heads and both are identical in other parameters, the disk with many heads will be faster. Splitting swap and putting it on both disks will be even faster, though.