>>42640691
LTO is some GCC compiler optimization, same as graphite. It's mostly autism, so if you're with the default march=native and O2 useflags keep at it it's fine. though I recommend you don't use -march=native but -march=Ryzen or whatever the wiki says your processor needs. as native needs gcc to guess and that's extra work for nothing
Everything I sent you can put in your make.conf as is.
Portage niceness uses Unix nice to not run every compile job at full priority, so you can use your computer comfortably as it compiles stuff for you.
The features one is mostly quality of life and eye candy.