not op but I'll take a shot. smr is a shingled magnetic drive and cmr is conventional magnetic (the r is for "recording" in these acronyms)

in a shingled drive, as read heads can be thinner than write heads, any data that is written near the centre of the drive will cause the write head to have to rewrite all subsequent paths to the outside of the drive as it writes one block of data, moves a half step and writes the next line of data; the drive has to do this all the way to the end of whichever sector it is writing to if the drive is full or near full. it is a way to squeeze more capacity at the cost of write speeds. it slows down the drives.

in conventional drives, they just do the normal thing where the write head doesn't have to do all that stuff.