Speed is the most important thing.
SEO, or Search Engine Optimization was the act of doing odd things to websites to make them appear more relevant to search requests. Trickery like white text on a white background, meta descriptions that went for miles, the odd "backlink" strageies.. The list goes on. Effectively it was the act of fooling search engines into believeing yours was the most relevant site.
Well now, speed is key.
All the way back in 2010 Google announced speed as a ranking tool. Does it effect every page terribly? No, not really, but it's a ranking factor, right next to how many people actually go to the site.. Recently, Facebook said something similar.
If signals indicate the webpage will load quickly, the link to that webpage might appear higher in your feed.
So if the bounce rate is high because there is a 10 second load.. Lower ranking. Not mobile friendly? Bounce away, reduction in site ranking.
Speed is being nice to your user's old device.
Check out this video.
In it, some guy from Google tells us that we are putting a ton of pressure on users of mobile devices. Not because we don't care, but because our laptops are way faster than our phones. I like to keep around a few old, terrible devices that don't have much use so that I can test my websites on it. If the load time is decent there, the load time will be decent just about anywhere.
All those transitions and cool flashiness needs to be able to work too. We can't just load tons of libraries into an app just because we can.
We have to think of what is important, and remove all of the noise that keeps our users in the way of that. Time is fleeting, and attention spans are, erm...
Because speed is more than just load time.
Speed is about how quickly the user can get what they need. Speed is about design, about removing fluff and accomplishing the users goals efficiently. Speed is load time, and speed is being mobile friendly. Speed is light on the unnecessary, and compressing the things that you need so that they are legible, but can fit through the tiny pipes of the internet.
We cannot pretend that our resources are limitless and every user is running a top of the line computer. Nor can we belittle the experience of those people who are. Our apps must be flexible and kind, focused and delightful.
Speed is saying no to almost everything