Opening up Google Analytics you are presented with a bunch of stats about your site’s traffic. What shouldn't be underestimated is the amount of influence website speed has on these values.
Pages/Visit
When visiting a snappy site like Amazon, you don't grudge clicking to view a related item. You know you won't be waiting long for content to appear. In a set time period, the less time spent loading, the more ground covered.
Bounce Rate
Bounce rate is the percentage of visitors who hit your site but leave, only having viewed one page.
Google can use your bounce rate in its ranking. A site might be completely relevant to what you were looking for but if its sluggishness is forcing visitors to hit the back button, it's understandable when you get knocked down a place.
Visitors and % New Visitors
Google looks at your site's speed as quality indicator. You'd prefer the faster of two identical sites, so Google factors this into their ranking algorithm.
If your customers leave satisfied they'll tell a friend, tweet, share on Facebook, blog etc. Lots of links leads to lots of new visitors.
Returning Visitors
A frustrating experience is remembered, making me think twice before clicking again next time your site appears on Google. Inversely, I positive experience gets you bookmarked.
Why is #onedirection's website so slow, driving me crazy???!!! how to move on if we never break down, #iUltimateQuotes
— Donnie Withers (@TpYoshiko) October 13, 2013
One less fan.
Avg. Visit Duration
Like me, at some point you'll have visited Amazon to quickly check the price of a new watch of something and before you know it you've burned twenty minutes and your basket is full of stuff from every other department. Nice snappy pages suck you in. Similarly see YouTube and Wikipedia.
Conversion
Your visitors will only dedicate so long to your site. The less time spent loading, the more time left to explore and absorb your content, leading to your finish line regardless of whether that's a checkout or new user signup.
Speed is a indicator of quality and therefore trust. You'll need to have this established this by the time you're asking for card details.