Generally, the message there is a result of our internal checking for response times related to what we expect. We added quite a bit of this back in Server 5.5 (and the SDK Response Time Observability) to help narrow the scope when performance issues are reported. So, that message on it’s own isn’t the cause, it just helps to correlate to determine cause.
If you’re seeing this alongside compaction, I might ask if you’re using ext4 as the filesystem? We have found that, not through any fault of Couchbase but just how ext4 and IO scheduling in Linux work, there is a big lock when using ext4 that may prevent other threads which are not involved in that IO from proceeding. Using xfs, at least as of when I did some investigation on this, seems to be the best fix.