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I'm a contractor at a rocket launch service provider. The final build of the ground control software is compiled and deployed to the launch pad with debug flags enabled because of a "fly like you test" mandate.
Millions of dollars and tons of time invested by brilliant people are riding on rockets that are launched using software with debug flags because of an "if it ain't broke don't fix it" mentality and archaic test strategies.
I've worked on ground systems and it's actually come in handy two times in five years, usually where we had a hard-to-reproduce bug. Getting the info when the problem happens can occasionally be all the difference.
Addendum: And usually we didn't care about performance. Basically never.