[quickthrower2]

Well I agree, you don't fire the person because they put JSON.parse(...) even if they put it in 1000 times. That would be silly.

The question is WHY did they do that? I'd probably get them to learn about performance tuning and do some profiling, make something faster. When they find out that it's slow because of something they didn't predict, hopefully they'll decide for themselves that they can't predict what will be slow, so no point complicating the code. If they don't get that, maybe explain it to them.

Basically the person who put JSON.parse all over the code was learning.

If they come back and arrogantly say "I'm right, and I'll carry on doing it you won't stop me", then that could be an attitude problem that might lead to question if they should be working there.

There are more nuances like if the person is claiming to be a senior developer/architect then the trigger for firing them might be more likely to be pulled. But still it is worth thinking about it first.