With a new install from a package, installed Couchbase Server 6.5. Added the travel-sample bucket. Left the system idle for approximately two days. This is <35k documents and about 6 indexes, IIRC.
Idle system is, well, idle.
Indexer process has regular CPU usage not commensurate with system activity. In fact, memory is not commensurate either.
I did not do any go profiling, but basic top sorted by time and strace -c -f -p are attached. The strace indicates for ~10s of wall clock time, there is ~7s of futex() syscall time. About 8% of those futex() syscalls end in error.