I'm not Zed either, but it's worth noting that you can usually do this without having to throw away the higher-level language entirely - most scripting language

I'm not Zed either, but it's worth noting that you can usually do this without having to throw away the higher-level language entirely - most scripting languages have C APIs so that you can profile and move the expensive parts to C. Sometimes it's necessary to change the architecture of the entire project, but more localized changes are often good enough. Lua and Tcl were designed with this approach in mind (they expect to be embedded), but you can do it with Python, Ruby, Lisp, Erlang, etc. too.