[WetWired]

The activity mod should have no effect on a search results page -- only where activity is displayed might it need to be calculated. Additionally, activity is only calculated once per user per day, the first time it is needed; if the post was made today, then the activity was already calculated when it was posted.

As far as optimizations, the default settings produce the minimum load, with the exception that more weeks counted will increase the load and that defaults to 4. Be sure to check the state of the "Show activity today for all users" option; this adds a query for each user who posted on a thread page every time it is viewed, which will result in a corespondingly large load increase. Except for that, the mod is designed so that a board with round-the-clock visitors will only feel the increased load once a day, and you can place that load off-peak by changing the mod midnight.