Hi all,
we are testing https/spdy in our websites, and are viewing strange results with an increasing time in the TTFB in spdy server resources as images in the https version.
For example:
https version: http://www.webpagetest.org/result/150114_RV_X7N/1/details/
http version: http://www.webpagetest.org/result/150114_A8_X7G/1/details/
The spdy version download the files a lot faster, but in exchange for an equal increase in the TTFB.
Is this normal? Thanks in advance, Simon.
Long TTFB is normal for SPDY because it will request all of the resources as soon as it knows about them and the server will stream the responses down (some times interleaving, some times back-to-back, depends on the server).
With HTTP(S) the browser sends one request to the server at a time and the TTFB is literally the amount of time it takes for the server to start responding to that one request.
There is a lot of wiggle room for server tuning/optimization with SPDY so the performance is going to depend a lot on the server implementation. In particular when it comes to supporting priorities (a lot of the SPDY server implementations do not). The browser will tag each request with a priority with css and js usually having a higher priority than images. A server that supports prioritization will deliver those first, regardless of when they were requested (assuming the file is quickly available).
In your case it looks like some images got served before some of the CSS files which would be really bad for rendering performance and is a sign that prioritization may not be working.
Thanks for such a great answer pmeenan, it was crystal clear ;).
I will look into this prioritization thing… probably is something related with nginx.
Thanks again for your help, Simon.