The delay looks to be because Apple has decided to speed up the multitasking and make the animation a lot smoother. I remember on iOS10, quitting an app was instant, however accessing the multitasking by double clicking would often lead to the app quitting before showing multitasking, or sometimes you’d get a half animation of an app closing before the multitasking showed.
Now in iOS11, in typical Apple fashion, the animation of multitasking runs flawlessly, at the expense of delaying the single press action (home) until the system knows you definitely aren’t going to double-click. Seriously, try double-clicking and tapping your open apps a few times, it’s a LOT quicker.
I suppose this is what happens when you have 3 different uses for a single button (home, multitasking and siri).