Improve performance of build expiry process

Recently we found our Bamboo server disk space hitting particular free space thresholds. We configured stricter artifact retention policies for some of our more artifact-heavy plans, and increased base restrictions for the global expiry.
While we can see the build expiry process running, it seems to taking more than a week to get through a full build expiry run, even though we've set Bamboo to run build expiry every night.
The expiry process needs to be more performant. It seems to be a serial process, so multi-threading it would be a start.
Additionally, instead of an expiry process kicking off at the global expiry time, it might be a possibility to have each build run the plan expiry process at the end of the build.