This article is from the CD-Recordable FAQ, by Andy McFadden (email@example.com) with numerous contributions by others.
The only possible basis in fact for the, "if it was recorded at 2x, you
can't read it faster than 2x" rumors is that some drives have trouble
reading CD-R media. Discs that are hard to read when spinning at 12x may
become easier to read when spinning at 4x. It has also been noted that
some recorders will write more legible discs at certain speeds (e.g. the
Yamaha CDR-100, which works better at 1x or 4x than it does at 2x). None
of this should lead anyone to conclude, however, that the write speed and
read speed are tied directly together. The reader, writer, and media all
have a role in determining how quickly a CD-R can spin and be readable.
It's also the case that discs written at high speed (say 8x) can be read by
drives *slower* than 8x. So if you're distributing discs to people with
old 4x CD-ROM drives, you don't have to worry about them not being able to
read at 8x. Of course, if the CD-ROM is poorly constructed, or the writer
is producing marginal discs at high speeds, you might see evidence to the
contrary, but there is no technical barrier to reading discs recorded at
8x or 12x on a slower drive.