Both, there's at least one commit that directly affects the performance of LLVM (improves caching, AIUI), and the patches that improve runtime performance will

Both, there's at least one commit that directly affects the performance of LLVM (improves caching, AIUI), and the patches that improve runtime performance will improve compile time too: the rust compiler is written in Rust and so benefits from the LLVM tweaks. (However, I believe both are relatively small.)