According to this, https://community.extremenetworks.com/extreme/topics/dfes_snmp_mib_walk_runs_slow_when_encountering_timefiltered_oids
Some OIDs used in the snmp discovery/list resources are time-filtered as per RFC3512, RFC4502, in which entries must return all valid timeMarks. So the NPM discovery always hits its default timeout of 60 minutes.
Also, using various MIB browsers that aren't timefiltered OID aware will also hang on OIDs such as lldpRemChassisId which I believe is required for the Solarwinds topology poller etc.
To fix our problem in which we experienced the same problem as yours, we entered the following command on our Enterasys S8:
set snmp timefilter break enable
hope that helps