I found this proverb in a translated article:「如果做某事太慢，還不如乾脆停下來。」Does anyone know its original form? I have searched several Chinese proverb sites but couldn't find anything remotely related to it.
The context is immensely helpful. I managed to find the source. It is from the book The building of The Burma Road by Tan Pei-Ying (譚伯英《打通滇緬路》). I found the excerpt in its original English version from Google Books.
Apparently the person who translated it into Chinese made a mistake. The English is the opposite of the translated Chinese. It seems that @dan's comment is quite right.
There are also many Chinese proverbs with similar meaning, eg 绳锯木断, 水滴石穿, and 只要功夫深，铁杵磨成针.
There is a saying: "一不做，二不休"(either don't do or don't stop).
It means "Either not doing it, or see it through to the end once you started"
In other words: "If you are not ready to finish what you started (too slow to finish), then don't do it (stop)"
Maybe it is the origin of this translated proverb
I really have not heard of such original proverb. But there is one really close, but it is a funny modern phrase, used in southern China or Hong Kong:
if the business (such as a mom and pop grocery store) is so low, we may as well play some mahjong or do a little bit of gambling
but this is the closest one I can think of, similar to what you wrote down as 如果做某事太慢，還不如乾脆停下來
There is another phrase that is close:
Either we don't do it, or we do it the best
but it has different connotation as your phrase because your phrase is suggesting stop when things are slow, while the phrase above is more like choosing between not doing it or doing it the best.