Hello! "on a Mac " is a rather vague information. OS version? Rig?
I just can tell you that is not usual at all; I have Painter X installed in a Macbook Pro C2D 2.16 gh and a Powerbook G4 1.5 gh (rather old , under current standards) and they run Painter smoothy. Tell him to fully uninstall Painter (including preference files) and /or to restore the default config.
Restoring the default Corel Painter settings
Corel Painter is designed to run from a locked volume (such as a read-only disk image or a CD) or from within a limited-user mode. All user settings are saved to a local user folder; when Corel Painter starts, it recalls these user settings rather than the application settings. At any time, you can restore the Corel Painter workspace to its default factory settings by replacing the settings in the user folder with copies of the settings from the originating read-only installation.
Restoring the Corel Painter workspace to its default factory settings removes all modifications you have made to the application including the brushes, papers, textures, and similar customizations. The locations of the user folders are as follows:
Mac OS /Users/<username>/Library/Application Support/Corel Painter X/
Windows \Documents and Settings<username> \Application Data\Corel Painter X
Restoring Corel Painter to its default factory settings is particularly useful in a multiuser environment, where many users are working from the same installation.
Corel Painter references the location of the loaded libraries for Papers and Color Sets. If Corel Painter cannot locate a loaded library, it reverts to the default library. When a new library is loaded, the user is prompted to decide whether to append to or overwrite the current library.
To restore Corel Painter to its default factory settings
1
Hold Shift when you start Corel Painter.
A warning appears, asking you to verify that you want to erase all of the modifications that you have made to Corel Painter. Restoring the default factory settings copies the original workspace settings from the installation to the user folder.
2
Choose whether you want to restore the current workspace, or all workspaces.