thats an over simplification.
10 seconds by itself does not mean DXM. 10 seconds to grey/black with no trace of purple and an initial puff of smoke may mean DXM.
the speed of the reaction is no real indication of the pills contents. certain other innert ingredients in the pills, such as filler and binder, seem to slow a reaction down. MDMA powder gives a very consistent fast reaction, almost instantly, but some strong MDMA pills, such as the CK series, give a very slow reaction, even tho they are kick ass pills.
the important thing to look for is the purple/blue colour change.