Memory speed
Memory speed (or speed grade) is a value representing the maximum 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 indicated on the chip. It is actually a time value (speed = 1/t) specified in nanoseconds or tens 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 faster, 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, some 120ns chips 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).
Note the possible confusion. If the value is less than 30 you certainly have to add a 0, even if SRAM is now available at 25ns (not 250) speed grades.
DRAM
DRAM (CD systems) is way more complex than SRAM, timings and sequences are primordial.