Personally, I'd be more focused on proper form than quantity and speed. Not that they are necessarily mutually exclusive, but it often works out that way in the real world.
For example, the linked article in the OP advises not to sag the hips while doing the exercise. However, the photo shows at least 2 guys with sagging hips during the movement:
Number 75 is sagging as well as the second guy from the right in the red shirt. Sagging makes the movement much easier. From personal experience, I know that you have to be very conscious of not sagging at the hips towards the end of the set if you are going all out. It takes effort to hold the proper plank throughout the exercise and it also makes the movement harder, as it should be. The guy at the far right of the photo has it right.
http://www.builtlean.com/2011/02/23/how-to-proper-push-up-form/