[dekimir]

> it's not clear to me why GLSL-to-SPIRV would be so slow. Would be great to hear back from Google.

Off the top of my head:

- glslang supports all previous GLSL versions and profiles, which adds many checks and corner cases throughout the code

- it's tricky to generate correct SPIR-V; for instance, it took me a while to properly understand the constraints around basic-block ordering

Disclaimer: I left Google in 2016 and haven't looked at glslang/shaderc since. I am only offering personal opinions here.