Speaking for the 767.
Cruise has been reduced from Cost Index 40 to 20.
Descent has been slowed from 290kts to Econ speed, usually 265kts.
Savings from this are quite small on an individual flight basis, but across a whole fleet on an annual basis can be quite large.
And yes, we 'waste' far far more fuel due to congestion than we could ever hope to save by slowing down.
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