Hello,
I have the same issues, 5-7 FPS on Mac running with eGPU Nvidia GTX1070, even on clear sky. All my other planes working well so I fly them at 25-50FPS with a settings: HDR, scenery shadows, textures high.
I found out that low FPS only for TBM900 is caused by HDR rendering in graphics setting. I tried set non HDR (one step back on slider in settings) and I got 45-55FPS. But this setting (HDR turned off) is not acceptable to me due to the cost of the TBM900 product.
@Cameron Thank you for the amazing plane, but please fix the tragic FPS in HDR settings to make the product worth it!