This makes a large difference :) Could we enable it by default?
Patch: https://archives.gentoo.org/gentoo-portage-dev/message/e0fd650bf286a027fa9fc0f6af7d8474
Patch v2 documents the mechanism of increased IO throughput, and adds a warning about the risk of undetected file collisions: https://archives.gentoo.org/gentoo-portage-dev/message/bf95852beabf96c5c564b405b552859d https://github.com/gentoo/portage/pull/550
Since the merge-wait FEATURES setting add for bug 663324 and enabled by default, you'll have to disable merge-wait for parallel-install to be effective with portage-3.0.62. With a few changes it's possible to uncouple them, allowing a batch of queued merges to be installed in parallel.