Thanks for sharing the name of your map. I can confirm that it is normal for the drone to slow down to 4 - 7 mph speeds when the 3D Mode is enabled. Please keep in mind that 3D Mode is intended for capturing details of a structure (ex. building) and it is not optimized for larger areas. Your mission will be longer and take more battery resources when 3D Mode is on.
Hope this clarifies it for you!