Getting my MacBook Air (2011) into Windows faster
I have a 20-inch mid 2007 iMac running OS X 10.10.5 and Windows 10 Pro 64 bit. Both were cleanly installed. This means before installing there was no previous version of ether operating system and the partitions were reformatted. Now, I would expect my boot times to be longer than yours. The strange thing is both operating systems take the same amount of time to boot. The time is about 64 seconds. Both operating systems are set to auto boot directly into my account.

If your Windows 10 is activated, then you can preform a clean install of Windows 10 using the latest Boot Camp drivers available for your Mac. Once connected to the internet, the new install will activate. You do not need a product key.

It is my understanding that Windows 10 skips certain steps during boot that previous operating system preformed. This allows for a faster boot time. You can still instruct Windows 10 to preform these skipped steps at the cost of speed. I don't know what happens when upgrading from Windows 7.

I first upgraded from Windows 8.1 which did allow me to active Windows 10, but I had enough problems with the upgrade that it was worth doing a clean install. I did not use the Boot Camp Assistant.

Windows is using a legacy BIOS boot. I doubt my computer would support an EFI boot of Windows. Although, I can boot Ubuntu (linux) using EFI without any problems.