SNMP is a fairly complex protocol, and the fact that it primarily operates over UDP does not exactly help matters. As a consequence, there are a number of potential problems that affect NMIS' ability to collect information from SNMP agents efficiently and quickly.
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The default for snmp_max_msg_size is 1472 bytes, just below the 1500 byte packet limit for normal Ethernets. In LAN-only scenarios it is possible to increase this past 1500 bytes: this causes IP fragments and packet reassembly, but unless your LAN is saturated and starving for bandwidth fragmentation is not a problem. The benefit of a larger SNMP packet would be that the data to be collected fits into fewer fewer packets.
max_repetitions
This option was added in NMIS 8.5G. It controls how many SNMP PDUs will be packaged into a single SNMP packet. The max_repetitions setting is named a bit oddly - that comes from the SNMP module that NMIS uses: Net::SNMP calls it "-maxrepetitions".
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If you observe SNMP error messages in the logs which look similar to "SNMP ERROR (X) (Y) The message size exceeded the buffer maxMsgSize of N", then you should set a lower max_repetitions value (or increase the snmp_max_msg_size if you're operating in a LAN-only scenario). Otherwise, a value of 40-50 minimizes the number of SNMP packets and thus speeds up collection. Not setting this option at all leaves it to the Net::SNMP module to guess a suitable value.
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