Zoompf's Web Performance Blog

Note: Archived Content

This is the archived version of the Zoompf blog. Since our acquisition by Rigor, all our new research and posts on web performance are being published on The Rigor Blog

Zoompf Check #300! Or: Gateway’s got a problem…

 Billy Hoffman on March 4, 2010. Category: announcements, Optimization

People often ask how we discover new performance issues. Without a doubt the #1 way we discover new issues is simply by looking at websites and seeing how they work. Not a customer engagement goes by where we don’t find at least one new web performance issue that we add to our growing database of web performance issues. This why Zoompf has added over 150 performance checks since we went public with our offerings. In this blog post we are going to give you the back story around a cool performance problem we found. In fact, it was so interesting and impact it became our 300th check in our database of web performance issues!

Picture of a cork popping out of a bottle

The Strange Case of CSS Resources

We recently made a few changes to our CSS parser and we were testing the new features against a few dozen pages to ensure we hadn’t broken anything. While testing, we noticed something odd on Gateway’s website. All of the background images in one of the style sheets for gateway.com were served over an encrypted SSL connection. In other words, the style sheet http://www.gateway.com/css/cms/styles.css is served over a plain unencrypted HTTP connection. However it includes links to background images like https://cdn.gateway.com/media/cphp/themes/default_bg.gif which are served over an encrypted SSL connection. What’s odd is the web server will serve those background images just fine if you request them using HTTP instead of HTTPs. It’s even weirder since very little of Gateway’s website even uses SSL. In fact, trying to access the root of the Gateway’s website using HTTPS will redirect you to the HTTP version.

So, certainly something weird, but is this a performance issue? After all, the developers just added a few extra “s” characters to their CSS. So what if maybe a little bit of SSL gets used. What are the performance implications of that?

Actually, they are huge!

SSL Primer

HTTPS is HTTP traffic sent over an encrypted SSL tunnel. SSL is expensive for 2 reasons (documented here and here). The first is the SSL handshake, where it is negotiated between the client and the server what protocols will be used and keys are exchanged. This involves the use of asymmetric key encryption with is extremely math heavy and quite slow. Then there is bulk data encryption, which is where the server is using symmetric key encryption. Symmetric key encryption is much faster than asymmetric key encryption, but can be much slower than sending unencrypted data. (How much slower is beyond the scope of this article. Suffice it to say that the performance impact of SSL is sufficiently large that there is an entire market for SSL acceleration products).

So what’s the impact in this situation? Well, the extremely expensive initial SSL negotiation must be done for the two initial connections to the web server. Each of these negotiations takes between 250 milliseconds to 350 milliseconds. This is in addition to the overhead of making the TCP connection and the negative impact of TCP’s Slow start and congestion control. Even reusing pre-negotiated keys a connection close is expensive, typically costing between 60 milliseconds and 100 milliseconds.

The Performance Impact

That doesn’t sound too bad but see how it affects Gateway. Examining this waterfall graph for www.gateway.com shows that 1.5 seconds, or over 25% of the page load time for Gateway.com, is due to the overhead of SSL! It’s hard to believe but the simple additional of 14 “s” characters to the page caused 1.5 seconds of delay! Further more Gateway’s web server is going to be working about twice as hard to push encrypt and serve those resources! Server load, power, cooling, and availability are all adversely affected.

(Notice there are a lot of other problems with Gateway’s website that contributed to the damage the SSL issue caused. Specifically they were not using persistent connection which caused a dozen of the smaller re-use negotiations to occur. Luckily Zoompf Check #177 and #178 detected all these closed persistent connections!).

Wow. So how did this happen? It could be for a lot of reasons, all of them fairly simple to make or innocent. It could have been a simple typo. It could have been left over from a redesign where that style sheet was used exclusively inside of the SSL portion of the site. It could be that this style sheet is used in both the SSL and non-SSL portions of the site, but to avoid a mixed content warning, the resources are referenced using SSL. The solution to this is simple. The style sheet should use protocol relative URLs to reference the images. That way if the style site is in the SSL portion of the website (and referenced using an https URL) the background images will requested using HTTPS. And when the style sheet is in the non-SSL portion of the website (and referenced using an http URL) the background images will be requested using HTTP.

It is both cool and scary that such a simple issue can have huge performance implications. And unfortunately none of the free tools like YSlow or PageSpeed would have alerted you to the issue. Until now.

It is with great pleasure and pride that we announce Zoompf Check #300: SSL Resources on Non-SSL Page. That this is such a simple issue to have and at the same time it causes such as massively negative impact makes it worthy to be our 300th check.

Want to see what performance problems your website has? Finding SSL Resource on Non-SSL pagesis just one of the 300 issues Zoompf detects when analyzing your web applications. Get your instant free mini web performance assessment at Zoompf.com today!


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