The above is the most likely but such a drop in FPS is unheard of except in this example.....
How 'dirty' is your map? Do you have large amounts of plot lines on your Nav Map?
It has been shown in the past that long/many plot lines/information/scribbles on the map can & will bring your game to its knees.
There is also a situation where shipping sunk in shallow water is not completley un-rendered by the SH3 engine. This means that even though you can't see them anymore, the engine is still tracking/rendering them. No fix for this though.
More likely my first suggestion. As it explains the gradual slow downs the further you get into the patrol.
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