In most projects I don't see any -Ox flags, which you think would be standard for every project, since it could dramatically increase the speed of the program.
Are there any specific reasons to not compile with -Ox or it's non-gcc counterparts?
It's much easier to debug an unoptimised program, because the object code tends to be a more direct translation of the source code. With optimisation enabled the compiler might re-order statements or eliminate them entirely by coalescing several operations into one. That means when debugging a program (or a core dump) there isn't such a direct mapping from a position in the program image to a line of source code.
GCC 4.8 adds a new optimisation level that is a great compromise between performance and debuggability:
A new general optimization level,
-Og, has been introduced. It addresses the need for fast compilation and a superior debugging experience while providing a reasonable level of runtime performance. Overall experience for development should be better than the default optimization level-O0.
With -Og the compiler does simple optimisations that don't make it harder to debug and don't take too long to compile, so the code performs better than completely unoptimised code but can still be debugged.