It took a minute to compile plan9 kernel from scratch on the original raspberry pi (running plan9). You can even cross compile a x86 kernel in similar time. 10 seconds in 9vx emulator running on FreeBSD/amd64. I don’t recall the details now but a from-scratch Linux kernel compile was 10 or 11 hours (under Linux on the same raspberry pi). Thank goodness it wasn’t written in C++; the compile time would’ve been so much worse!
C compiler optimizations seems like micro-optimizations when people should be looking at the bloat elsewhere. Missing the forest for the trees.
C is basically a low level language. A portable assembly language. A predictable compiler shouldn’t second guess the programmer’s intent. To put things in perspective, if all the man-years spent on gcc were spent on GNU Hurd... :-)