How has it suddenly become so much faster to wake OS X from "safe sleep"?

If you have Filevault enabled and don't get the EFI authentication screen (apple.com > HT204156), you're not waking from standby. You should be prompted for creds twice: once by EFI for Filevault, then a progress bar as the Filevault volume is mounted and hibernation image restored, then a second authentication from the OS to log back in.

sudo pmset -f log may help to confirm what's going on.

For what it's worth, I've also found that the hibernatemode=25 option seems broken. Testing on a 2013 MacbookAir6,2 / A1466 running 10.11.5 with hibernatemode=25 (direct-to-standby, skip sleep), the machine did hibernate on closing the lid after I'd first enabled this option. However it's not done it again. (I did update to 10.11.5 in the interim...)

Apple's documentation https://support.apple.com/en-us/HT202124 says, "Notebooks need to be running on battery power [and] must not have any external connections (such as Ethernet, USB, Thunderbolt, SD card, displays, Bluetooth, and so on)" in order to go into standby.

Paying attention to the above conditions, I've tested this a few times and my machine will only go into standby (and thus, destroy the Filevault key if destroyfvkeyonstandby is enabled) when the battery is fully depleted.

i.e. hibernatemode=25 doesn't actually work as documented in pmset(1).