Memory speed
Memory speed (or speed grade) is a general concept in electronics, and a value representing the time it takes for a memory chip to output valid data after the address is changed.
It is essential for memory chips to be fast enough to avoid crashes or glitches. When replacing RAMs or making cartmods, care must be taken to use chips with appropriate speed grades.
Speed is usually part of the chip's markings. It is actually a time value (speed = 1/t) specified in tens or hundreds of nanoseconds.
During the specified time, the memory data output isn't guaranteed to be valid. So if the system expects data to be valid too soon, it might catch wrong bits resulting in unstable operation.
Regarding the NeoGeo, faster (lower value) isn't a problem, same speed is fine, slower (higher value) is bad. A slower chip can work in some cases, but you should never expect stable operation in all conditions. For example, a 120ns chip could work in place of a 100ns one at 5.1V, but cause an immediate crash if voltage goes below 5.05V. Trust the manufacturer's specifications.
Standard speed grades
-250 (250ns), -200 or -20 (200ns), -150 or -15 (150ns), -120 or -12 (120ns), -90 (90ns), -70 (70ns), -55 (55ns), -35 (35ns).