Steam Long Load Times With Dx12 On Pc Lenovo Legion T7 10th Gen I7

If I add -dx12 to the launch options in Steam, when I run TSW4 the Train Sim World 4 logo appears in the middle of the screen, and stays there for a long time. On the last launch, it took 12 minutes for the title screen to show and the music to start, and then a further minute or so to get to the intro and game menu.
If I remove -dx12 the game launches immediately.

Specs are:
Lenovo Legion T7 Desktop PC running Windows 11 Pro 64 bit
Intel Core i7 10700K @ 3.80GHz (overclocked to 4.5GHz)
32GB DDR4 @1599MHz
MSI GeForce RTX 3080 (not ti)
Steam librrary is installed on a Samsung super-speed USB 3.0 SSD

Anyone experience anything similar?