the 'trick' is to do a clean Win10 install with an an USB Stick made from the MS Win10 Media creation tool, BUT for the first but, you need to press the Option key and choose the native USB Stick and not the legacy (you should see 3 options: MacOS EFI, USB EFI and USB Bios but symbols, no text)
Win10 installs extremely fast in 20 Minutes. AND the boot time is reduced from 15 seconds to 1-2.
The BootCamp Assistant USB Stick, which does not support EFI Boot, at least in my case with 2011 MBA, is needed for the post installation work.
But: as of Sept. 21 there was no working video driver from intel, screen keeps getting dark, and Windows Update also forces download and install of that bogus driver. be carefull.
for me only the generic video driver worked, but does not support any standby/suspend modes and no sound.
what DOES work: install Win10 in non GPT BIOS/legacy mode, for some reasons(???) the video driver then works and the rest and only this annoying BIOS Wait time even after hibernate.