Long story short: yes, there is a glaring, technical issue with the decals that are causing this performance issue. It can easily be fixed as I will outline, but it should be removed from the pool and labeled with performance issues until addressed.
I am not at my PC right now, but will show why this is happening later by looking at the map draw distances for decals in the editing tool.
Mavvy's marsh has EVERY decal at a maximum, allowable draw distance (5000 or greater game units of height) while most maps are about 300-500 (basically "zoomed-out" view while in game). The engine has a hard time processing all of this information and is not meant to be pushed to this limit.
I recommend removing it and sending a note to Chosen to fix it, be given a chance to address the issue (finally).
There are methods for "macro" decals where you can have certain, enlarged decals drawn while zoomed out, but not in. I recommend he does this if he is unhappy with the visual degradation.