I'm working on profiling integration, and I got some first numbers of performance comparison of our shader pipeline on a real-ish project: https://github.com/kvark/vange-rs. This is just about creating 2 shader modules (from SPIR-V!) and a render pipeline. Very primitive and artificial test, take the numbers with a grain of salt!
SPIRV-Cross: 5.1ms
Naga: 3.0ms
Common part (calling Metal routines): 1.55ms
That leaves us with a rough upper bound on the time spent in Naga and SPIRV-Cross respectively being 1.45ms and 3.55ms. This tells us Naga is 2.5x faster, roughly, at least with regards to how it's used in wgpu tech stack.