Ever since I've updated to Sketch 4 and beyond it became slow and causing performance issues. Loading times are painfully slow and it doesn't matter if there are lots or only a couple of artboards, images etc.... Even zooming and panning across became slow. Using a Mac mini late 2014 with 2,6GHz i5, 16gb of memory, ssd hard drive and Intel Iris graphics, running El Capitan 10.11.1.
I do not have performance problems with Os X nor any other graphics software, even Adobe stuff runs smoothly.
Anyone experiencing the same issues?