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’.