Users are only accessing the Community server through their Notes client so the SSC, Meetings etc don't get involved at this point.
I have seen this with some customers and a restart will sort it out eventually. You should enable LDAP trace (VP_LDAP_TRACE=1) as the overhead is low and look for warnings in the stresolve*.txt files relating to "pending" queries. What I believe is happening is that the Community server has a back log of stresolve queries it wants to send to LDAP and it can't get enough sent and returned from LDAP.
If you have never tuned LDAP then you should look to. This doesn't necessarily involve touching LDAP (unless that is the cause and it isn't turning around queries quickly enough) but rather adding settings to the Community server.
The following is a good guide http://www-10.lotus.com/ldd/stwiki.nsf/dx/Best_Practices_for_using_LDAP_with_Lotus_Sametime
Look at setting the following:
ST_DB_LDAP_MIN_WILDCARD=<Minimum number of characters> suggest 3
ST_DB_LDAP_MAX_RESULTS=<Maximum number of results> suggest 100
Then look at white listing so the Community server will only send email addresses to LDAP which fit your domains. This avoids the Community server from sending hotmail and yahoo addresses which you know won't be resolved to a person in LDAP.
I have seen many LDAP related problems for 40000+ user companies to smaller 300 user ones.