Well, I suppose it is, compared to the processor on your desktop today.
But compare it to the first computer with cache, a pipeline, a Williams tree multiplier, and an advanced division algorithm - no, not the 66 MHz 5-volt Pentium, but the IBM System/360 Model 195 from 1969.
Enormous, room-filling, and the top-of-the-line computer from the world's best-known big computer company. Significantly more powerful than the previous 360/95 computers that NASA had.
What were its specs?
16.67 MHz clock.
Upgradeable to four megabytes of RAM. 64-bit bus to the RAM, running at 1.32 MHz.
But you could fill the sixteen megabyte address space (architecture like a Pentium, address space like a 286...) with significantly slower bulk core; something in the neighborhood of 100 kHz - or, as they would have said in those days, 10 microsecond cycle time.
I know this may be like talking about how we went to school in the snow, uphill both ways, when we were kids, but being a few years behind the times (perhaps it's state of the art for a rad-hard processor) shouldn't stop Curiosity's computer from being able to do great things.