Q: What's a cEM? What are cEMs/sec?
A: A cEM is an acronym for "Corrected ExpMod". An "ExpMod" is a somewhat misleading measurement -- it represents a single iteration of the modular exponentiation algorithm used by the client. This shouldn't be confused with an entire exponentiation, since it requires exactly n - 1 "ExpMod"s (and a few insignificant extra operations) to complete a single Proth test. The correction factor comes into play since the time needed to complete a single ExpMod increases as the size of the numbers we're testing increases. A cExpMod takes this into account, and at n = 300000, exactly 90 cExpMods equal 1 ExpMod. At n = 400000, 160 cExpMods equal 1 ExpMod. Confused yet? :-)