Merge two UIImages into one
On a simulator with the original code, it takes about 1.52s on my machine. 

Since the base image won't get resized (self.size is passed in self.draw(in:)), and its alpha channel is always 1, I could gain at least 200ms by using the following :

self.draw(at: CGPoint.zero, blendMode: .copy, alpha: 1)