I'd argue it wasn't, given than less than a year after they released the console in Japan they started stuffing carts with extra chips to make up for the processor performance shortfall. I'm not sure why they chose the Ricoh 5A22 but it wasn't for it's performance, there were rumors that they were targeting NES backwards compatibility until late in the console's development and the choice of CPU (among other hardware quirks, like the controller signaling) fits into that.

Not to say that the SNES wasn't a 2D powerhouse thanks to it's powerful raster effects and developers eventually got better at working around the anemic CPU, but earlier games chugged, particularly if publishers cheaped out and used slower ROMs for the carts which cut the already slow CPU speed from 3.58mhz down to 2.68mhz. Mode 7 was a neat trick, but on the whole the core SNES hardware was really bad at 3D.