when it comes to ...; as for ...
Used to introduce a topic, often with a nuance of criticism, complaint, or strong emphasis. Followed by a statement about the topic.
When it comes to him, he's always late.
As for our cat, it does nothing but sleep all day.
となると introduces a hypothetical or conditional topic, whereas ときたら presents a known topic for comment.
Derived from the conditional form of と来る (to come), literally 'if it comes to ...', which developed into a fixed expression for introducing a topic.