¹slow

delay, retard, slow, slacken, detain mean to cause to be late or behind in movement or progress. delay implies a holding back, usually by interference, from completion or arrival <bad weather delayed our arrival>. retard suggests reduction of speed without actual stopping <language barriers retarded their progress>. slow and slacken also imply a reduction of speed, slow often suggesting deliberate intention <medication slowed the patient's heart rate>, slacken an easing up or relaxing of power or effort <on hot days runners slacken their pace>. detain implies a holding back beyond a reasonable or appointed time <unexpected business had detained her>.

delay, procrastinate, lag, loiter, dawdle, dally mean to move or act slowly so as to fall behind. delay usually implies a putting off (as a beginning or departure) <we cannot delay any longer>. procrastinate implies blameworthy delay especially through laziness or apathy <procrastinates about making decisions>. lag implies failure to maintain a speed set by others <lagging behind in technology>. loiter and dawdle imply delay while in progress, especially in walking, but dawdle more clearly suggests an aimless wasting of time <loitered at several store windows> <children dawdling on their way home from school>. dally suggests delay through trifling or vacillation when promptness is necessary <stop dallying and get to work>.

You can slow up and you can slow down. Either way, you're doing the same thing: you're going slower.