What might prevent ubershaders from working?

[themanuel]

I started playing MP3 (MPT), which is notorious for shader compilation, but with async ubershaders enabled.  I get huge stuttering and long pauses when new shaders are cached, but excellent performance otherwise.  

Are there any known system or GPU settings that might interfere with ubershaders?
How could I diagnose this?

Thanks in advance.

[JosJuice]

If you are using an Nvidia GPU, switch to one of the Direct3D backends.

[themanuel]

Sorry, I should have specified. I have been testing with both, D3D11 (mainly) and Vulkan, with same results.