と言（い）えば, と言（い）うと, and と言（い）ったら are all expressions that are translated quite closely to ‘speaking of (A)’, or ‘if it were the case that (A)’. Each of these structures has a slightly different nuance, but are largely used in the same situations.
と言（い）えば - More hypothetical, ‘were it (A)’.
と言（い）うと - More concrete, ‘if it’s (A)’.
と言（い）ったら - More situational, ‘when it’s (A)’.
These constructions are most commonly used after nouns, in response to something that has already been mentioned in the conversation (usually by someone else). The thing that has already been said is quite often the ‘trigger’ for someone remembering something, causing them to interject with と言（い）えば.
If we’re talking summer, it’s gotta be a Suika-bar, don't you think? (Suika-bars are the best thing when it is summer, don't you think)
Speaking of Tanaka-san, he is the person that got arrested for robbing a bank right?
Ramen? If it’s Japan, sushi is more popular, isn't it?
Although these phrases are used in response to something that has already been mentioned, their purpose is to change the topic of the conversation somehow (or to add new information), rather than continuing along with the same subject.
Speaking of summer, since it’s a festival, do you want to go to the festival tomorrow?
Speaking of Kikuchi-kun, I wonder how old he is.