Old English slāw 'slow-witted, sluggish', of Germanic origin
The word slow is normally used as an adjective (a slow learner; the journey was slow). It is also used as an adverb in certain specific contexts, including compounds such as slow-acting and slow-moving and in the expression go slow. Other adverbial use is informal and usually regarded as nonstandard, as in he drives too slow and go as slow as you can. In such contexts, standard English uses slowly instead. The use of slow and slowly in this respect contrasts with the use of fast, which is completely standard in use as both an adjective and an adverb; there is no word ‘fastly’.