Apple recommends and supports only 120ns or faster RAM chips for the
Macintosh II.
In theory, the Macintosh II does not operate properly with SIMMs
containing RAM chips slower than 120ns. This is because the number of RAM
wait states defined in ROM does not change dynamically to accommodate
slower RAM. (This is not an issue with the Macintosh Plus and Macintosh SE
because their slower processor clock speed accomodates slower RAM speeds.)
In practice, however, slower RAM chips (150ns, for example) will work on
a Macintosh II because RAM chips are often faster than their labelled
ratings. Nevertheless, always use RAM that is tested and rated at the
required speed of the computer. Otherwise, you may eventually experience
random system crashes or other inconsistent errors using SIMMs slower than
120ns.
(NOTE: Macintosh Plus and Macintosh SE do NOT dynamically allocate wait
states according to memory speed.)