Well... Now I know the answer. Throw a bucket of money on it. The 2080Ti loads up that scenario at 40+ FPS and never looks back.

Then, I maxed out water quality, shadow quality, tessellation factor, and increased the LOD radius from medium to large. FPS dropped to 33-36FPS.  I switched to 8xSSAA and it's holding at 25-28. Not too shabby for 0630 @ KBOS with DL and lots of AI. Deactivating HDR did not add any FPS to any of the 3 configurations.

I still feel like the driver was doing something funky with the older HW that it's not doing with the newer. It was "stuck" trying to do something with HDR and changing the setting somehow got it going again.

Seemed to be confused by the setting somehow. Latest spec. GPU does the job and more.

I was gong to suggest trying a factory default, but too late, you've achieved that by fitting the new card. Very nice too.

 

So... It happens with the 2080Ti as well. I left P3D running sitting a C40 in DEN for several hours. When I came back she was sitting at 6-8 FPS. Nothing I tried unstuck it. Due to user error, I was not able to save the flight.