[rich! @ etiviti]

i would just store a last_activity (every 5 minutes – like the standard last_activity) for each group a user is a member of then run the compare on the topic->topic_time (maybe to make it cleaner – store the last_activity for each group in the bb_meta table – and restrict it down to is_forum and is_forum_topic )

(WP and BP use a lot of cookies as it is – you run the danger of some of the being purged out when the limit is hit)