If you experience overheating and performance issues of your old MBP while drawing, try the following:
Close any instance of Rebelle.
Go to Applications Folder, rightclick Rebelle and click Get Info ( or select Rebelle by left click and press Command+I )
In the App Info window expand General drop down menu and tick Open in Low Resolution option. That's it.
The same thing works for Photoshop.
It doesn't affect displaying of your artwork.
You can revert this setting back by unticking this option any time if you don't find it useful.