It's not as much about RPM as the way the sectors are laid out on current drives.

In the early days, every sector took up the same angular space on a disk. That meant that inner sectors were small, and outer sectors were larger. Zone Bit Recording put more sectors at the outer portions of the disk, where there was more space per track. To do this, the drive electronics fools the computer into thinking that it has the same number of sectors per track. So the computer can think it's putting a file on contiguous sectors, while the drive is putting them in different places. Sector sparing automatically replaces bad sectors with good ones from spare cylinders, further reducing your chances of getting true defragmentation.

As drive capacity grew beyond the limitations of the original PC BIOS specification, a technique called "logical block addressing" was invented to essentially do away with the cylinder-head-sector addressing that was done in the past. With LBA you essentially have a bunch of sequentially numbered allocation units, which is what the computer sees. Physically, on the disk itself, two consecutively numbered sectors aren't necessarily next to each other on the platter. LBA still uses CHS for the sake of compatibility, but the logical CHS structure (what the computer sees) is nothing like the real structure on the disk.

The result of all of the above is that defragmentation has become a lot less effective. Modern drives and operating systems use caches to buffer disk reads and writes, making defragmentation less of a performance factor.

P.S. Glad you like PageDefrag. I use it wherever I can.