This option enables Clang's full Link Time Optimization (LTO), which
allows the compiler to optimize the kernel globally. If you enable
this option, the compiler generates LLVM bitcode instead of ELF
object files, and the actual compilation from bitcode happens at
the LTO link step, which may take several minutes depending on the
kernel configuration. More information can be found from LLVM's
documentation:

https://llvm.org/docs/LinkTimeOptimization.html

During link time, this option can use a large amount of RAM, and
may take much longer than the ThinLTO option.