For GUI's I don't see this as an issue - JITs run alot faster than humans. All of Javascript is on a JIT - and thats the dominant UI for now! For servers generally you find that users are all calling the same function so it's very rare that you hit a blip - much more commonly users get performance problems from something else in the stack like the network or the client, and while Javascript is the dominant front end the dominant back end for servers is Java and that's got a JIT as well.

JavaScript's JIT is a tracing JIT, so it can compile code in the background while the interpreter/less optimized compiled code is actually running. In Julia, the compiler runs first, and then the compiled code is run. This will probably eventually change as Julia's compiler improves, but regardless, it's important to note this distinction.

It can make use of JIT caches (if it doesn't already), not every JIT compiler starts from zero.