If you're not sure if those logs are related, perhaps you should run
"date" when things get slow and find the relevant debug logs that way?
I didn't see any smoking gun in what you posted, but since you're posting
about "slow" with a log describing filter processing I'd look into
unindexed searches. You're using syncrepl, so hopefully you read in the
slapo-syncprov man page
On databases that support inequality indexing, it is helpful
to set an eq index on the entryCSN attribute when using this
overlay.
and followed that advice? If you added indices, did you run slapindex
after stopping slapd? Look for messages along the lines of