That's right shadowlin, I was not aware of this GUI extension of the official wallet, it all makes sense now, thanks a lot !

But from what I see on the forum topic you linked, this GUI uses Janror's CPU plotter, which is what I used to create my plot files that were not optimized... But maybe they made some changes when integrating it into that GUI so I'll give it a try.

And thanks Ahead for your kind explanations