It is fairly common knowledge that server performance helps with the SEO viability of a website. This is never more evident than when you upgrade server hardware to a better server. You can see the effects almost instantly in Google Webmaster Tools. The stats will show that your pages are being crawled more because it’s taking less time.
The image above is from a website of mine showing my average stats before and after a server upgrade. I move this site from being a Windows hosted WordPress site to being a Linux hosted WordPress site. My performance on this Windows host for my WordPress sites was continually degrading and got to the point where I didn’t even want to use my sites anymore. Now this isn’t to say hosting a WordPress site on a Windows server is a bad idea. It just has to be configured correctly for it, In my case it was not configured correctly for it and being on shared hosting there wasn’t much I could do about it.
This is a shining example of why it’s important to make sure that your web sites are hosted on the best available hardware for what the site requires. It became beneficial for me to switch to a Linux host so that I had WordPress hosted in their natural environment. As you can see this caused my download times to drastically decrease and caused the number of pages crawled by Google to drastically increase.
Now this was a simple change to make to have it make such a big impact so fast, but it is only one of the things that factors in a site’s SEO viability. So while site performance is very important for it’s SEO and user experience values it’s not going to replace other SEO methods.