In src/liboslexec/llvm_instance.cpp, OSL uses a custom list of 13 LLVM optimizer passes (plus implicit passes pulled in as dependencies) -- less than what Clang uses at -O2 for C, but still a fair amount. Then it runs the full LLVM "-O2" backend. Even if the compilation happens "dynamically" from the perspective of the application, OSL is using many compiler features more commonly associated with "static" compilation.

JIT speed is relative. Shader programs are often executed many many many times, so OSL can likely afford to make different tradeoffs than many other JIT-using applications.