If I have PORTAGE_NICENESS set to 10, and do a world update which includes a new portage version, it restarts the update (as expected) after upgrading portage. But since the niceness is additive, it ends up doing the rest of the update at niceness 19. Not normally much of a problem, unless you run Boinc (seti@home, etc.) or other distributed computing tasks. In that case, portage fights 50/50 for CPU power and the update takes forever.
*** This bug has been marked as a duplicate of 44541 ***
Okay, so it's not a duplicate. I'll fix it.
This has been fixed in svn r3336.
Thank you!
This has been released in 2.1_rc1.