A new smart filter for IRC join/part/quit message has been added. It is disabled by default (so you'll see all join/part/quit by default).

To enable it, use following commands:
/set irc.look.smart_filter on
/filter add irc_smart * irc_smart_filter *

You can setup delay: /set irc.look.smart_filter_delay 5

How does it work?

When a nick spoke on channel during past X minutes (where X is delay irc.look.smart_filter_delay), its join/part/quit will be displayed by default. Otherwise, if nick did not speak for a long time, all join/part/quit messages will be tagged with "irc_smart_filter". So you have to define a filter on this tag to hide join/part/quit from users that are not speaking on channel (see command above).