There are somethings which are not expressly said but implied. For example, when you scroll down the list of apps in cellular settings, the list stutters and this list has been stuttering since iOS 9. Its been 2 releases and its never been fixed. This proves Apple is not interested in fixing it. You are not going to hear Apple come out and expressly say the home button delay wont ber fixed. Its communicated through the releases
This is easy to prove. This may seem like malice to us customers but it makes perfect sense from Apple's viewpoint. The majority of iPhones are bought when trhe phone os being hyped and people are reading about it in reviews everywhere. If you will observe the trend of all iOS releases in the past, older devices are slowed down but as future versions are released things improve, but the performance improves only once the maximum sales in the release quarter are realised.
Apple wants everyone to buy the X. Its their true flagship. Therefore they simply slowed the 7 down a little so that when customers play with an iPhone X or an Android phone (Apple won't like this one) they will realise just how much faster and more responsive it is. The counterpoint here is why would Apple release the 8? Apple can't make enough of the X to sell them which is where the 8 comes in but Apple knows that the X is still going to end up cannibalising the 8 and those who buy the X are in far greater numbers than the 8 and they would still prefer it if the customer orders a X and waits till January/Feb to get it instead of getting the 8 as it means higher margins. But Apple didnt want to take a risk here that those with the older phones and who cant get the X this year because of delays have nothing to look forward to this year which is why the 8 was released but Apple still slowed down the button of the 8 because they are dangling the ultra fast X in front of those owners so they get the the X at its higher price next year.
iPhone 7 enjoyed the luxury of being the flagship phone for a year which is why Apple gave it the love it deserved in the form of an ultra fast home button. The 8 is the middle step child no one wants sadly and will fade into irrelevance like the 5c once X releases