with sys-apps/portage-2.2_rc4 emerging tuxonice-sources with prelink installed seemed to cause it to check every file (prelink --verify), which is a major waste of time, since the kernel sources don't contain anything that gets compiled. Reproducible: Always Steps to Reproduce:
The "prelink --verify" thing should be a mute point as long as you have FEAUTRES="unmerge-orphans" enabled, and it's enabled by default since portage-2.1.3. When the unmerge-orphans feature is enabled, portage will unmerge files without verifying the checksum (unless they happen to be protected by CONFIG_PROTECT).
I committed the changes to prelink as described in bug #235971. Sync and rebuild your prelink once the change hits the mirrors and test again. The /etc/prelink.conf file now contains -b /usr/src which will blacklist this directory.