Hi,
Fallbacking to another LDAP server is done by OpenLDAP internally. So what would be needed is either a) OpenLDAP to itself figure out that queries are running too slowly and see if another server is faster, or b) Dovecot figure that out itself and force OpenLDAP to switch to another server.
I don't know if either of them is possible with OpenLDAP API as it is. I doubt it.
You can set a timelimit for a search, but I'm not exactly sure what happens when you specified multiple servers. Maybe the query gets restarted on the next server, maybe it fails. Still doing that (with logging) would be better than hanging there for several minutes.
If it completely aborts the query, a workaround would be to use more than one ldap passdb with one server each, no?
Bernhard