It's the CPU's of old... The first real IBM PC ran the 8086 processor, which didn't last long, and was replaced by the 8088 which they got rather more mileage out of. It went more or less like this: (I fergit the 'zact years. Summa those are kinda foggy fer me.)
8086 (late 70's, early 80's)
8088 (mid 80's)
80286 (late 80's)
80386 (early 90's)
80486 (mid 90's)
Pentium (mid 90's)
Pentium II (late 90's
Pentium III, IV... blah blah blah...
Now the 4Ghz P4 is pushing more than a thousand times as fast as those early 8088's.
Amazing.
I can't imagine what speeds will be achieved by the time I have grandchildren.
Wooo-WEE!