When I run Snap Art 4 on my Macbook's built in Retina display, it's slower than when I use my external monitor.
This is happening because of the pixel density of the Retina display. In order to show the preview image, Snap Art has to render four times as many pixels as it would on a non-Retina display. This higher resolution image looks much better, but it takes more time to render.
If you prefer to prioritize speed over image quality, you can disable Retina support in Snap Art's preferences dialog.
For those of you who aren't quite familiar with what Retina means, when referring to a Mac, here is a simple example. You can see the non-Retina example on the left, and the Retina example on the right. Retina displays have four times as many pixels, which means that it takes four times as much work to render the image, as it would on a regular, non-Retina monitor.