It's worth mentioning that even with studies no one really knows how Google works their speed factors into the rankings. I know many sites that are extremely slow and still rank because the sheer amount of authority, backlinks and domain age these kinda sites have.
So while you should try to get a fast site for your audience its not always a winning factor.
Your site is 1216ms which is extremely fast for a shared hosting account, but I question this results and you will find that speed checkers online vary from site to site because the location of their servers vary in location and the checks they do do not proper emulator the actual speed of the average broadband user.
If your not already I advise you to check http://www.webpagetest.org/ and do tests using the 5mbit since well in the UK 3-6mbits is the average. So find out the average speed of your TARGET country and then use that as a standard. Most like 5mbit since 20mbit won't be average unless your in sweden or china or something ;)
Furthermore, tests all servers in your region and outside, chances that 1216ms that your reporting is actually higher unless your shared hosting is actually rather good. It's also worth noting you can open lots of tabs and run multiple tests along side each other to actually emulator real time visitors because after all when your doing a TEST rather than tests your only testing one current user which is no way a realistic TEST.
There are 2 major factors which slow down websites, these are:
1) Server Byte Speed (The time it takes for the server to say, HELLO! I got your request - heres the information) Most shared hosting accounts will add 800ms-1000ms on the server byte while on VPS they are much lower,.
2) The amount of requests on the page (This means if you have 20 images, 20 java-scripts then you have 40 requests and ultimately lowering this number will reduce the time to takes to render.)
Having Good Byte time and fewer requests is all what you should focus on, your notice that these speed tests do lots of tests and rank your site on many factors - well the one I listed does :P its in my honest opinion the best out of their due to the options of multiple sites, multiple tests, multiple speed emulation.
I hugely debate wpengine.com outcome on this matter and very much doubt for each 100ms, Google will apply these factors in place in segments along with over changes, many factors change rankings all the time there is simply no way no one can say for sure. And as I said many TOP SITES operate between 1-2seconds. Also and I know this message is getting long - Google does not TEST servers for LOAD so a site that is doing 1.2secs might be 5secs if its a 1,000 users, and a site that is 1.5secs might stay at this speed with 20,000 users. They test all the time but factor in many of the ranking factors which just because they lost rankings when they increased the speed doesn't mean it was a direct result of the speed.
Assuming that your site is 1216ms on the site that I've mentioned and your happy with it. 1216ms is absolutely fine by my standards, Google standards and most likely everyone who replies to this thread ;)