Different behaviour to DX11 when vsync is disabled in Editor

To reproduce:
1. Open repro project 

2. Open Window > General >Test Runner and double-click the test 'PerformanceTest'
3. Open Window > Analysis > Performance Test Report and click 'Refresh'
4. Observe the median FPS

Expected result: Median FPS is similar between DirectX11 and DirectX12
Actual result: Median FPS is lower on DX12 compared to DX11
(From local test:
DX11 ~320 FPS,
DX12 ~120 FPS)