Specular reflection models differ in sophistication (more sophisticated = more render time). These different implementations solve different problems with varying degrees of performance impact.

However the differences in performance have long ago dropped out of the matter. Modern computers implement sines and cosines just about as fast as add and subtract.

Left to right, anisotropic, phong, glossy, blinn, specular.

Of the five, only blinn is moderately accurate in the backlit case.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)