Kore Nordmann has written up some benchmarks of some of the more popular PHP image creation libraries testing their output times and results for generating the same image.
In my article series I showed several ways on how to create images with PHP, going into implementation details etc. Now I just want to provide a very rough overview on speed and quality of the extensions.
He ran them with some settings disabled to optimize things as they ran and came up with results pointing to Cairo being the fastest to generate and output the image. It also had the benefit of outputting an image with the highest quality of the five tested (Cairo, SVG, Flash and GD with and without supersampling). His benchmark code is also included.