Google has stated that they won't penalize unless your site is in the bottom few percent. So if you are under 10 seconds are so, you probably won't get a specific algorithmic penalty.
Rankings also appear to go down if the site is too slow for users to find it usable. The conventional wisdom here is about 3 seconds. If you can get your page load time under 3 seconds, few users will turn back when clicking your site from the SERPs, and your rankings will improve as a result.
Keep in mind, users may find your site USABLE well before it officially loads completely. Take the case of a video site where it takes 30 seconds to buffer the entire 2 minute video, but a user can start watching it even while it is buffering.
Your official load time also may be slowed by 3rd party elements that don't hinder the user at all. I've seen cases where tracking pixels and analytics scripts at the bottom of the page take 10 seconds and the user is none the wiser.
Msg#: 4498725 posted 8:23 am on Sep 23, 2012 (gmt 0)
I recently started experimenting with a cdn. Before using it on any part of the site our rankings were on the up, I implemented it for banner images across the site and product images for one section and within 2 weeks rankings for that section began dropping slightly. At the last Panda update rankings fell across the site by a few places.
It doesn't make sense that the cdn would cause this, but it is my key suspect at the moment. We had issues initially with browser caching - the cdn servers were overriding our cache-control settings, then our hosts enabled mod_expires which caused more havoc which we think is now sorted. One issue we were never able to resolve is that it seems impossible to control Expires dates for cdn images (at least that's what my hosts told me!).
We have a UK site on a US server, the hosts are the best company I've ever used and much cheaper than UK companies so I thought a cdn would be the next best solution to moving to a UK server.
My hosts use a third party for the cdn however, and I don't feel so confident about them or the cdn to be honest.
Whenever I've done something that slowed a page up, even by a couple of seconds, I've seen rankings drop within a couple of weeks. This makes me suspicious that our cdn is not quite providing the speed benefits we hoped for.
I've used some top tier CDNs in the past like Akamai and EdgeCast on a fairly big site. No change in ranking that I could attribute to the CDN usage.
A CDN is only really needed if you want better response times in regions that your main server is not located in. When we used Akamai, our US visitors didn't notice a difference in speed compared to serving content directly from our Softlayer server. Buying a CDN to improve Google rankings is a bit misguided I think, I've seen a few CDN providers advertise this as a bullet point but there are other ways to improve site speed that have a far more measurable impact. Consolidating small images into a single image using CSS sprites is a biggie.