The original OSL Shading System API was stateful: You'd create a shader
group, configure it, and then end it. However, this means that only one
group can be created at a time. Further, since Cycles reuses the
Shading System for multiple instances (e.g. viewport render and
material preview), a process-wide mutex is needed.
However, for years now OSL has had a better interface, where you
explicitly provide the group you refer to. With this, we can not only
get rid of the mutex, but actually multi-thread the shader setup even
within one instance.
Realistically, most time is still spent in the JIT stage, but it's
still better than nothing.
Pull Request: https://projects.blender.org/blender/blender/pulls/130133
d1796b8df0