To improve security and to prevent file descriptor leaks, snmpd always closes all its filedescriptors when processing 'extend' or 'pass' configuration options. Doing so, it blindly closes all filedescriptors lower than current resource limit of opened files (ulimit -n, /proc/sys/fs/file-max).
When these limits are high, it may take some time to close these filedescriptors and snmpd thus slowly reacts to GET request in NET-SNMP-EXTEND-MIB::nsExtendOutput1Table.
snmpd does not read resource limits from operating system, instead it finds out on its own which file descriptors it uses.
Requests in NET-SNMP-EXTEND-MIB::nsExtendOutput1Table are much faster.