Apple is expected to fix their VM deficiencies in late '97 or early '98. There are a few aspects of hard drive performance. Generally bigger drives are faster because they have more data crammed into the same space. So if all the mechanisms move at the same speed, then they are able to access more data in the same amount of time if that data is more closely packed. So bigger is often better.
Some people only measure seek as from one track to the next - but that is an ideal condition.
This time delay is what is usually referred to when talking about drive latency. RPM's - the less this latency. Transfer rate - once the drive gets to the data you want, how fast can it send the data to the computer. So the faster the connection and faster the drive is spinning with the most data in the same amount of space - then the better transfer rate you will get. Cache - how much of a buffer the drive has built in to make it SEEM like it is faster than it really it.