'Slow' originally meant 'blunt, torpid' — like a dulled blade. The speed sense came later.
Moving or operating at a low speed; taking a long time; not quick to understand; (of a clock) behind the correct time.
From Old English 'slāw' meaning 'sluggish, inactive, torpid, lazy,' from Proto-Germanic *slēwaz or *slaiwaz, meaning 'blunt, dull, sluggish.' The original meaning was closer to 'sluggish, torpid, inert' than to the modern 'not fast.' Related to Old Norse 'slær' (blunt, dull) and Old High German
'Slow' and 'sloth' are related — both come from the same Old English root 'slāw' (sluggish, torpid). The deadly sin of sloth is literally the sin of slowness, of torpid inaction. The animal called the 'sloth' was named by Spanish explorers who observed its extraordinarily slow movements