When everything runs normal, the game does not care if the mirror window is fullscreen, borderless or windowed. Either should work fine. You can try resetting your Elite graphics settings by removing or renaming (the latter is probably better) the folder
%AppData%\Local\Frontier Developments\Elite Dangerous\Options\Graphics
and removing / renaming the gpuworktable.xml, which is in the subfolder of whatever version you play in the installation's Products subfolder. This forces ED to reprofile your GPU.
Other than that, the usual:
- Check and maybe reduce SteamVR render scale
- what GPU are you running? Are you possibly running out of GPU memory?
Maybe reinstall SteamVR, or switch back to the last known good version (this is under betas).
Those are all the ideas I have off the top of my head. Good luck.