I've seen quite a few people mentioning the fact that you see all topics in the active topics page, even those in forums you may not access. Personally I found that a greater problem then some speed loss with an extra permission check. So I made a little mod 
The following you'll have to change in active_topics.asp:
On line 261 add the following after directly topic , " & strDbTable & "Permissions "
Insert after line 262 the following line:
strSQL = strSQL & "AND (" & strDbTable & "Forum.Forum_ID = " & strDbTable & "Permissions.Forum_ID AND " & strDbTable & "Permissions.Group_ID = " & intGroupID & " AND " & strDbTable & "Permissions.[Read] = TRUE )"
One requirement for this to work correctly is that you must set the permissions for all groups. Only grouppermissions will be looked at.
Sorry for the SQL users but this is Access only.