At certain times of the day (usually late day/dusk or early morning/dawn), if you have this set to 4096 it will drop your performance by as much as 60-70%.  I recommend using a value of 2048 or less.

 

If you would like to confirm this problem you can download the two saved flights and weather files (be sure to disable any weather engines  you have running): http://www.robainscough.com/uploads/Prepar3D_v2_Files.zip

 

The setting in questions is in your Prepar3D.cfg (located in i.e. C:\Users\Rob\AppData\Roaming\Lockheed Martin\Prepar3D v2):

 

[GRAPHICS]

GROUND_SHADOW_TEXTURE_SIZE=2048

 

Cheers, Rob.

 

EDIT: Corrected path.

Heck, I find that shadows look more real as the resolution goes down.  I think mine are at 256.

Not at my computer, but what is the default size?

2048 I think