@Goeasy
I agree with 99% of what you just said. There is no way of knowing how slowing down affected the ride unless we fly thru the same (theoretically and practically impossible) pocket of turbulence 2 times at two different airspeeds.
No question, at optimum cruise altitudes we cant slow down significantly. Sometimes not at all.
In my airplane, if we fly at optimum altitude .80 above FL430, we are already at Vref or very close to it so there is no margin to slow down anyway.
However, if you depart Shanghai to Tokyo, Chinese will keep you at FL260 for 250nm. That is where we have an option to fly 320 knots or as slow as 240 knots at moderate weights and still have safe margins to stall (like Vref +70 or so).
@FE Hoppy
In my airplane turb pen speed is 240 below 10000ft and 270/.80 above
It doesnt take a rocket scientist to realise that it doesnt work like that: 240 knots at 9999ft and 270 knots at 10001ft.
For me personally, if I have a very good margin to Vref at current configuration and altitude with PAX, I will slow down below 270 KIAS for light to moderate and above. If it is ferry flight, I normally never slow down below
Vb.