In gentoo is it possible to build again a package, with the already extracted package? So for example the source is extracted at a location. I don't want it to be extracted again, but instead use the same location and add the necessary flags. Wouldn't it amount to lesser build time, when USE flag is changed?
I think you are looking for ccache which has a Gentoo Wiki article for getting it setup. This can definitely reduce compile times of hungry packages like chrome and libreoffice. Some additional info here. Note, there maybe issues when using distcc.