From a different game that had the same issue, the Dev is using 'show' instead of 'scene'... in my very limited renpy knowledge, show layers the image over the previous one while scene replaces it (or basically that - please forgive me knowledgeable people).I'm also having tremendous lag issues. And i have windows 10. I'm no computer expert, but according to what i'm seeing on my task manager, it nearly fills up the RAM. I can hardly make it through a couple renders before the game slows down ridiculously. It eventually gets to the point where i'm progressing through a line of dialogue only every 20 or so seconds. I've tried changing the Renderer. Not sure what else to do.
If you open the events.rpy and replace all the 'show' entries with 'scene' and save it. When you open again the increasing lag will not be present. You need to start a new game though.