SPI flash is usually NOR flash (not NAND like you'd find in an SD card or SSD). NOR flash is usually SLC and will take 100k writes/erases and is quite simple to access (as compared to MLC NAND which only usually takes 3k erases and is a pain to access comparatively due to things like ECC requirement).

SPI flash generally will do on the order of 0.5 MB/s erases, slightly faster writes, and somewhere in the realm of 4 to 12 MB/s reads, give or take. It's not high performance but it's simple, cheap, and low pin-count. The host controller is responsible for wear leveling, erasing each needed sector/block, and all writes, as SPI flash is unmanaged (compared to SD cards which are managed memory).

AFAIK, SD cards do not have wear leveling, SSD drives do.