Not sure if this will be any help but I have experienced low frame rates at times and believe it is related to add-on airport scenery which may not be totally compatible with 5.3. I found various solutions - sometimes deactivating the scenery and / or  re-installing or finding a more compatible airport package. My specific problem occurred at KRSW over Orbx True Earth Florida. After re-installing KRSW my FPS changed from about 5 or less to 27 and about 23 with VR.