Let's clear some things:
1) this plugin currently works with two different preclear scripts: the original one from Joe L., which I can't redistribute (see OP for more detailed info) and the one I wrote (previously called "Beta script"), which is included in the plugin by default. If you download the preclear disk script (Joe L. or bjp999) and put it on "/boot/config/plugins/preclear.disk/" you will be able to select between them when starting a preclear;
2) Performance hits: the new preclear script is, by design, faster than the original script wrote by Joe L. All performance hits are result of me decreasing the default read chunk size to 32MB. You can easily increase it by setting a higher number on the "Read Size" select box. I'll improve the code soon.