Slow Performance Flying Over Japan

I am using a couple of addon meshes from http://flightsim.com/zview.php?cm=list&fid=136275 which covers Australia, New Zealand (including Auckland Islands & Macquarie Island), PNG/Irian Jaya, Indonesia, Malaysia, Philippines, Taiwan, Japan, Korea, Micronesia, Solomon Islands, New Caledonia, Vanuatu, Fiji Islands and all other Pacific Islands.I have flown over the majority of these areas, and find FSX's performance degrades when flying over Japan only - i.e., it takes a few seconds to switch into the Map and Save screens. Initially, I thought this is because of dense sceneries in the cities, but the performance problems occur in non-populated areas as well.Do you know why is this and/or how to fix it?Cheers.