A just in time compiler is a way of compiling scripts or applications on the fly. It allows an operating system to interpret and compile code (or precompiled code) on the fly instead of having compiled code on the machine (during runtime).
How this interprets to the speed of an Android device? I'm a little fuzzy on those details. I suppose if the JIT compiler was faster than the binary files or the current method of execution (isn't it JIT anyways?), it would make it faster? It would definitely give an advantage to developers who wanted to code in various languages and expand the capabilities of a device (a good example would be that all Java is interpreted through a JIT compiler).
If the current method of execution is not JIT, and you are going to JIT, I would suspect you would experience lag at startup while the JIT compiler was loading into memory, and then the speed increase may be noticed at execution...