Size and Speed
I am aware of the relative lengths of the Sea King and the Chinook. However, comparing their silhouettes I would still say the Chinook is about twice the size of the Sea King. Perhaps bulk is a better word. A champagne flute may be taller than a half pint glass but the latter is bigger.
I am only trying to fathom out a reasonable explanation for the alleged change in speed of the Chinook. It seems very unlikely that it would slow down to 70 knots then accelerate to Vne as it approached the Mull.
If some people believe that this was the case are they trying to say that there was some fault with the engine control system, causing the aircraft to decelerate then accelerate uncontrollably, and that this was the cause of the accident? Perhaps distracting the crew at a critical time?
I think it is much more likely that it would be maintaintaining a steady speed. Perhaps 120 knots - two miles a minute, a very handy speed for navigational purposes, although it may well have been 140 knots.
As a fixed-wing pilot with little rotary experience I would certainly not describe even 140 knots as a dangerously high speed. At that speed the turning radius of any aircraft is very small and the Chinook should have been able to manoeuvre easily at turning points. I am sure someone with more rotary-wing experience will confirm or deny whether this is the case.