webpagetest.org is a mighty tool to test performance. An API documentation is mature and extended: https://sites.google.com/a/webpagetest.org/docs/advanced-features/webpagetest-restful-apis
There is a test tool based on Google Tables - i mean, it is a high time to get it into STFE 
Thanks, great suggestion!
How long would you say the tests take? Not sure if running a test should return id (to fetch results manually) or do polling until the results are ready. Too long wait time is not good.
By the way, did you see my PM about SimilarWeb?
did you see my PM about SimilarWeb
No, even checked my inbox - don't got it
How long would you say the tests take? Not sure if running a test should return id (to fetch results manually) or do polling until the results are ready. Too long wait time is not good.
Yes, i would divide getting test results in two parts: firstly just get a test id, than read out needed metrics from the raw result file.
The most cool way would be if STFE runs own private instance of WPT - on this way STFE customers can scale tests and STFE can take some bucks more for such cool service 
I'm the first who would pay for this
Haha, yeah that would like the Spider on steroids
Sent you a new mail, we've had some issues with the Forum servers but should be good now.
This is a really mighty tool. But even if it would have just one single function, it would be worth to use it: the absolutely unique function is the SpeedIndex, the loading time of above-the-fold area.
There are many approaches to measure above-the-fold loading time (i wrote down in german a small overview about methods), but this one is the most realiable and resilient.