Not exactly. Most people don't have iPhones older than the 4S and many people (myself included) with the 4S are not upgrading to iOS8 because iOS7 ran just decent enough on the 4S, plus many of us with 4S's are upgrading to the iPhone 6. iOS8 brings the 4S to a crawl according to reviewers on the internet.
That basically leaves iPhone 5/5S users upgrading to iOS8 which is a smaller amount of customers.
I installed 8 on a 4s yesterday and it has not slowed it down at all. I wanted to test it for my husband because he is not upgrading his phone.