Portage is excessively lethargic when uninstalling large volumes of packages at once, which is considerably normal when you're using FEATURES="test", especially in the Perl and Rust ecosystem. It would be very nice if portage respected the -j parameter for unmerge and depclean, so that tasks like verifying checksums and soforth and running pkg_post_rm can be properly parallelized. Naturally, this probably wants some sort of graph handling, so nodes are only cleaned once their dependents are also cleaned, so that interrupting a clean minimises system breakage. As it stands, I'd imagine there are groups of packages that take less time to install than it does to uninstall, due to install being parallel, and uninstall being purely linear.
This would be valuable for catalyst livecd builds, which spend quite a bit of time uninstalling packages at the end of the build.
+1 this will indeed speed-up tinderboxing