I noticed when testing with a Yum DB with a million records that when I ran the Yum Metadata Rebuild task manually, it fired off a log message saying starting in waiting super quickly:
2018-05-30 17:38:02,340-0400 INFO [qtp346300068-209] admin org.sonatype.nexus.quartz.internal.task.QuartzTaskInfo - Task 'setup' [repository.yum.rebuild.metadata] state change WAITING -> RUNNING
2018-05-30 17:38:02,361-0400 INFO [quartz-5-thread-6] *SYSTEM org.sonatype.nexus.repository.yum.internal.createrepo.task.YumCreateRepoTask - Task log: /Users/jtom/Documents/Work/nexus/NX3/nexus-internal/target/sonatype-work/nexus3/log/tasks/repository.yum.rebuild.metadata-20180530173802359.log
2018-05-30 17:38:02,362-0400 INFO [quartz-5-thread-6] *SYSTEM org.sonatype.nexus.quartz.internal.task.QuartzTaskInfo - Task 'setup' [repository.yum.rebuild.metadata] state change RUNNING -> WAITING (OK)
However, under the covers it's still running and the metadata isn't (re)generated til it's complete so it looks like it failed.
This seems to be pretty confusing for anyone who doesn't know this. I certainly thought it failed when the log said waiting and the metadata didn't show.