The release notes for gcc were a little vague on -Og:
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.
-Og the compiler has to construct and write out extra data (for debugging), so it will take longer. Just compile to assembler (with
gcc -S -Og, etc) and compare. But whatever difference there is between
-Og runtime is probably dwarfed by the time to start
gcc and its complete machinery.