With the 5D II and 7D the AF was slower with the V1 500mm than the 1D series. With the V2 500mm the 5Ds R and 7D II is noticeably slower than my 1D IV.
Why would it be slower with newer batteries and technology?
I think Fast Glass's theory would be easy enough to test.
Shoot without IS on see how many fps you get.
Shoot with AF off and IS off and see how many fps you get.
If it is still low I would give some credibility to FG's conspiracy theory.
Besides, Canon is in the business of selling new equipment.
Supporting lenses first released three decades ago might be counter productive.

Keeping in mind that the battery, AF and IS when the Version 1 500mm was replaces was just 10 fps with the 1D IV.