Having the same issue here. VR performance degraded completely after the release of 2.5.4.
Used to get 45 to 90fps with VR preset + terrain and cockpit texture settings turned to high + PD 1.5.
Now I'm lucky if I can get > 30fps with all settings turned to their lowest.
Non-VR performance is unaffected.
Tried every trick I've learnt in the last three years of playing DCS to no avail.
I'd actually given up on playing in VR entirely until I saw this thread.
Setup:
Oculus CV1
i7 4770K overclocked to 4.6GHz
16GB RAM
SSD
GTX 1080ti