I ran DOS-based AutoCAD on a 33 MHz 486 at work, 1991 through 1994. I felt privileged - most of the other machines were 20 MHz. If we zoomed too far out of a large drawing, the drawing would regen, which could take 10 minutes. Partway through '94, we got our first Pentium, 66 MHz. I was working on a huge system restoration drawing, which took almost 8 minutes to open on my 486. I copied it to a floppy (3.5"? 1.44 MB? Not sure - we still had lots of the 5-1/4"ers around too), had coworker load it onto the hard drive of the Pentium machine, and we both invoked the drawing at the same time. The Pentium opened it in just under 4 minutes, which was absolutely mind-boggling to us. Of course my 386 took its usual 8 minutes.
A few years later, I'll say early 1999, I saw a computer magazine in 7-11 with "500 MHz is coming!" splashed across the cover. Mind-boggling at the time.
In the classic Sci-Fi novel The Moon is a Harsh Mistress, published around 1966, author Robert Heinlein has Mike the supercomputer running at 12 MHz. I'm sure that was quite unbelievable to the computer-savvy readers of the day.