Yes, it's okay if it's started once, but if you have to run it over and over then it's not. For example run it 10000 times, and if time it takes 300ms - you are looking at an hour of execution. Some apps (even if they are big) are just naturally cached by the OS in the file system buffers, but others are doing quite of other things before running.

If this is going to be a "shell" replacement it needs to start very fast at least in hot mode (cached).