but I don't know at what extend the performance of the resulting executable is increased

It isn't, performance is decreased. The way that PAR works is by creating an executable that contains all of the files that will be necessary to run the script embedded in itself. At runtime, each of those files will be extracted into a temporary directory (I think they are extracted as needed, rather than all at once at the beginning, but I'm not sure). The overhead of extracting the files is going to add a bit of extra time to the running of the script, though I don't think it's enough to worry about if PAR solves other problems for you.