Considering he played into his 50s, it's not surprising he got slower! He wasn't a spinner as we would know them today. Medium pacers in those days would often bowl spin as a variation though. Primarily he bowled medium pace.
He was probably a similar type of bowler to the New Zealand player Chris Harris, who bowled slow medium pace with some occasional spin thrown in.
It should be remembered that he was a round arm bowler, and leg spin is pretty much impossible with a round arm action of the type Grace had.
Also, slow bowler does not always mean spinner.