Just stuffing in a higher rated CPU won't make it faster. During this era of computers the rating on the CPU was just an indication of what speed the manufacture was guaranteeing it would operate at under "reasonable" conditions. A 33Mhz CPU will run at 25Mhz unless you change the clock speed fed into it, and that may or may not be trivial on a NeXT Cube. (Seriously, no idea, never owned one.)

These days, of course, different speed-grade CPUs usually operate at some common bus speed while a (usually-)fixed multiplier inside the CPU dictates the core speed so your confusion is probably forgivable.