WideImage uses GD extension to perform most operations on images. Some operations aren’t available in GD (like applyMask), and are coded in pure PHP, which is slow, but does the work. WideImage introduces some overhead compared to specifically optimized code written directly with GD functions. This overhead is very small, and shouldn’t be a problem in most cases. It’s the price you pay for library’s flexibility and power. You may encounter memory consumption issue, though.
So, if you have mission-critical image transformations to perform on large images, and you have to do them really fast, WideImage is not the way to go. But the same goes for GD, and even PHP.