I feel like it should just end at "slow." Thats atleast the feel it gives off. "Why are you slow?" "이렇게 태어났거든"
That's not correct. It'd be asking why someone is mentally retarded.
Given a previous correct answer, the phrase indicates the lack of tact. This would imply that the person is curt, abrupt,or evem rude.
The subject is not given. It copuld just as easily been "Why am I slow witted." Asking a Korean this question to his or her face is about the biggest insult I can think of and would likely end any friendship or relationship. It is a 큰 일! You're calling them stupid!!!
The answer should be changed or the statement should be remomoved.