Was the Spey-engined `toom a hot-rod?

[Tailspin Turtle]

In McDonnell flight test in 1967, our tired F-4C chase plane had no problem keeping up with the first F-4K during acceleration at altitude. We (and Rolls-Royce) were relieved when it finally reached Mach 2. The Brits blamed the shortcoming on airframe drag and McAir, less than spec thrust. Performance may have improved subsequently.