You guys using different physics than the rest of the world??
Of course high speed below 10,000 saves time. Using 310 kts instead of 250 kts from 40 nm to 20 nm saves you .9 of minute.
Speed vs altitude is the name of the game. 250 kts at 10,000 at 40 miles to go, is a great efficient and quick energy fix. Crossing it at 310 kts means you'll have to use drag to get rid of the excess energy. Here's the solution - for every 30 additional knots be approx 1000' lower. IE - 280/9000/40 and 310/8000/40 are about the same total energy.
You can double check this by removing the FMC's 250/10000 restriction and inserting a fix at 40 miles to go. Then put the different descent speeds on the FMC's descent page and see how much lower you'll be for every additional 30 KIAS. It's about 1000'.
There are places and seasons where I don't do 'high speed' below 10,000. The majority of the time it's John Daly time - "grip it and rip it." (Don't overthink it, just do it)