There has always been a delay when pressing the home button (to allow for double and triple clicks) - if you are experiencing a particularly long delay then it may be worth checking the home button settings under accessibility
I've just done a comparison between my SE (iOS 11) and my daughters (iOS 10) and there is absolutely no perceivable difference in speed when closing an app.
I've just done a comparison between my SE (iOS 11) and my daughters (iOS 10) and there is absolutely no perceivable difference in speed when closing an app.