Is the multiple community strings has been set up under the Discovery settings , does it slow down the Discovery itself ?
If the communities fail, the discovery will be slower. If the first community you have matches, then next ones are not tried. If no community matches, it will go through all your communities list and all their timeouts-retries.
The SNMP timeouts are incremental. That is, if you have a 4 second timeout with 2 retries, the first try it will wait 4 seconds, second try it will wait 8 seconds and third try it will wait 16 seconds.
Refer to the following DOC for a detailed explanation of the discovery in LMS.