if you have to ask, then the likely answer is that csv imports are likely to be faster than what you can write

the more nuanced answer is that the premise that both csv import and map/reduce can do the same thing is flawed

for example, a scheduled import is something you would use a map/reduce for that you wouldnt use a csv import for

same thing in reverse, a one time import is something you would use a csv import for and not a map/reduce

there are more complications in that you can use suitescript to trigger csv imports, but thats more rare and i dont think that was what you were aiming for