As an adverb meaning 'slowly,' 徐 is classical and literary. In modern spoken Chinese, 慢慢 is used instead. 徐 as a surname is extremely common in modern use.
Left step radical, a small footstep, the half-form of 行 (to walk). Indexes 徐 in the motion-and-path family with 往 to go, 徒 on foot, 待 to wait, 行 to walk. The radical anchors the meaning: walking with deliberately small, unhurried steps.
Right 余 supplies the sound, yú drifting to xú via well-attested initial shift. Same phonetic in 途 way, 涂 to smear, 除 to remove. 余 originally pictured a thatched hut and now means 'remaining'; here it carries no meaning, but learners can hook the slowness onto 'taking one's time, with leisure to spare.'
