Why did you say you were fired for underperformance? Why didn't you say it was a bad fit? Usually, that's the root cause.
Also, mathattack's answer: https://news.ycombinator.com/user?id=mathattack
I have seen great people fired by terrible managers.
As an interviewer, I appreciate the honesty/brevity, and if the explanation for being fired is reasonable enough, it needn't be a huge red flag.