Steps to reproduce:
1. Open attached project "performancetest.zip"
2. Build for WSA Universal 10 SDK
3. Deploy on a Windows Phone 10 device with Adreno 305 GPU (eg. Lumia 640/720/1320)
4. Observe UI.Text displaying FPS in portrait mode
5. Turn the phone to landscape mode and observe FPS
Result: FPS drops from 25-50% depending on Unity version(due to increased rendering time)
Alternatively connect profiler and observe the change when switching from portrait to landscape

Notes:
Not reproducible with WSA Universal 8.1 SDK
Not reproducible with other WP10 with different GPU's

Reproducible with:
5.3.1f1, 5.4.3p3, 5.6.0a6 - 25-30% FPS drop
5.5.0p1 - about 50% drop (however by far better performance compared to other versions)

Tested with: Lumia 720, Lumia 1320, Lumia 640(user)