Is JIT Compiling Slower?

The JIT is looking for code that is going to be executed frequently, right? Like there is some heuristic about whether things are being rerun and therefore are applicable. I don't think traditionally that it would necessarily always create compiled code. But my impression of jitting is that really it'll interpret code and when it sees code that is hot, meaning that it's in a loop, well, then it'll generate machine code for that part. That should be a lot faster than if you need to have a side effect of leaving code behind.