In early versions of Java, when an applet was executed for the first time, the speed at which it executes was disappointing. The reason for this is that the Java applet's intermediate bytecode is interpreted by the Java Virtual Machine rather than compiled to native machine instructions as is the case for C++ as discussed previously.

One solution to this performance problem lies in Just-In-Time (JIT) Compilation (also called Dynamic Translation). The JIT compiler reads the bytecode of the Java applet and converts it into native machine instructions for the intended operating system, just after the applet is loaded from the disk. This can happen on a file-by-file basis or even on a method-by-method basis - hence the name, just-in-time. Once the Java applet is converted into native instructions, the application/applet then runs like it was natively compiled. The JIT compiler can, in certain cases, improve the run-time execution speed of applets by a factor of 5-10 times, while still providing portability through retaining the use of intermediate byte-code. Compiled code is kept in memory until the application terminates. In effect, when using just-in-time compilation Java applications are compiled twice, once when the source code is translated into platform portable bytecodes and again when bytecode is being executed on that platform.