speaking of ...; when it comes to ...; as for ...
Used to pick up a topic just mentioned and add a related comment. The literal meaning 'if one were to say ...' is rarely used in modern conversation; the phrase functions as a fixed topic marker.
Speaking of summer, it's definitely the beach, right?
As for him, I haven't seen him at all lately.
Grammaticalized from the conditional form of と言う (to iu, 'to say'), literally 'if one says ...'. Over time it lost its literal conditional meaning and became a fixed topic-introducing expression.