A lot of times code compiled with no optimisations (-O0) is unusable. Specifically, in video some software compiled without optimisations won't push frames on t

A lot of times code compiled with no optimisations (-O0) is unusable. Specifically, in video some software compiled without optimisations won't push frames on time and instead will just keep dropping frames. There was a post a couple days ago about it being problematic in the games industry where a game compiled without optimisations is unplayable, while higher optimisation levels are hard to inspect in a debugger, due to the myth of "zero-cost-abstractions" in C++. Also to put it on its head a bit, when a compiler isn't fast enough (read not enough work was put into performance of the compiler itself, mostly on the design level, not on the microoptimisation level really), the feedback loop is so long, that developers stop testing out hypotheses and instead try to do as much as possible in their heads, without verifying, only to avoid the cost of recompiling a project. Another instance: when a photo-editing application can't quickly give me a preview of the photo I'm editing, I'm going to test fewer possible edits and probably get a worse photo as a result. With websites, if an action doesn't happen within a couple seconds of me clicking I often assume the website doesn't work and just close it, even though I know there are a lot of crappy websites out there that are just this slow. Doesn't matter. The waiting usually isn't worth my time and frustration.