How long time does Process.start take?

When the file is 97Kb it, this is only the file that is reported back. In reality the file could already be much bigger but because the file is still open (and written to) it isn't updated to the actual size. Also, doesn't really matter because the file simply isn't done yet.

It would be faster to run the conversion of .gsm files like a service that converts files from one directory to another for example. The process is then resident in memory and ready to go when requested. Now you start it when needed and could be that it is already started multiple times by other users or not at all. Both could affect performance because there could already a lot of processes running or the jvm needs to start up. As a single service it is already started and only a single instance is going at a time. Maybe you could add multiprocessor support and add some threads that do the conversion but you stay in control of how many active conversions are executed.

A simple way to check if the conversion is ready, is to give the file some other extension. Simply converting somefile.gsm to somefile.wav, you could name it somefile.busy when still processing and rename it when complete. This way you can easily check if the file is done.

Good luck!