Monday, June 14, 2010

Optimizing Site Performance

Purpose & Goal

Right now I’m running some tests through to establish baselines for site performance. This week I hope to generate some CSS sprites for our common images and set up gzip and xpiration headers on the server. Those are the last 3 big things pointed out by YSlow (actually I’m using Google Page Speed, but YSlow is cool too). Hopefully soon I'll be able to run a new set of tests to compare to the baselines I'm running now.


I'm trying to get a representative sample of of pages to test. The home page gets the most traffic, followed by the Current Students page. Pride Online has the most popular departmental home page, but it's utilitarian and not representative of other pages. That moves us down to the home page for the library.

Those 3 pages realistically represent only about 1/6 of our total pages, but it’s the most heavily visited 1/6. I could add one more test case and increase my representativeness to the majority of our pages, but we’d see diminishing returns in terms of traffic (only about 16% of total site traffic). Also, the remaining 5/6 of pages are mostly text heavy and image lite. The benefits to those pages will come from optimizing the overall site template, which will be partially reflected in the 3 chosen test cases.

Thanks to Google Analytics for the ability to access that info easily.

The Site Performance widget in Google Webmaster Tools says we gained nearly half a second in our rendering time around June 6th. That half second was enough to move us up from around the 73rd percentile to the 82nd percentile. I have no idea why we saw this jump in overall site performance but I'm glad to have it. Maybe we can even hold onto it.

The Results Are (Mostly) In

The 3rd and final test is still running as I type this. I’ll link out to the results as they become available.

Step 1: CSS Sprites

I'm using SpriteMe and my own best judgement to create some CSS sprites for commonly used background images. I'm assuming SpriteMe already has something like built in.


Ok, maybe I need to smush my sprites after all. I've added about 60kb to each page load while freeing up about 15 http requests. can’t find anything to optimize, but the image optimizer in YSlow can shave about 5k total from 2 of the sprites. SpriteMe makes some less than optimal choices, even occasionally introducing bugs. But like all software it has to make certain assumptions and those don’t always pan out. I need to improve upon the default sprites while I’m at it. With these newly optimized sprites, let’s look at the trade offs.

jQuery UI Theme Icons for Play Controls

The first sprite I made was actually already made for me thanks to the jQuery UI Theme Roller. I used the blue and red versions of the UI sprite to replace the play control icons under the featured stories on the home page. The bad news is that almost doubles raw file size. In exchange, I pre-load a lot of UI elements that we're not currently using anywhere else. But I plan to make more use of the jQuery UI in the future. So I think this is a worthwhile change.

Baseline With Sprites Change
HTTP Requests 8 2 -6
Total Bytes 5,310 9,724 +4,414

Stripes Sprite

Both the header and the footer have background stripes that go all the way across the page. These were tiny, 1 pixel wide repeated images. But they were easy enough to combine.

Baseline With Sprites Change
HTTP Requests 2 1 -1
Total Bytes 905 122 -783

Image & Badge Drop Shadows

Some day I’ll be able to pull this off with pure CSS but in the meantime I’m stuck with background images. The one for the standard large image gets used on most pages. The one for smaller images at the same aspect ratio and the badges gets used fairly often too. The other 2 don’t see as much use. Some pages don’t use any of these, but the majority of our high traffic pages do. The image produced by SpriteMe left extra white space to the right of the 16px icons screwing up the placement. I had to edit the sprite in Photoshop. After that actually did a better job compressing the resulting image than YSlow.

Baseline With Sprites Change
HTTP Requests 4 1 -3
Total Bytes 4,018 5,092 +1,074


The image produced by SpriteMe left extra white space to the right of the 16px icons screwing up the placement. I had to edit the sprite in Photoshop. After that actually did a better job compressing the resulting image than YSlow.

Baseline With Sprites Change
HTTP Requests 18 1 -17
Total Bytes 14,589 13,677 -912


I combined our logo with the RODP logo that’s featured on every page. But that only nets us 1 less HTTP request and as you’ll see below costs us significant bytes due to the differences between .jpg and .png image formats.

Baseline With Sprites Change
HTTP Requests 2 1 -1
Total Bytes 12,270 39,876 +27,606

In a Nutshell

In a best case, a single page that references all these images (which is not terribly realistic), we’ve traded 28 fewer HTTP requests for 31,399 more bytes. That’s about 22 additional HTTP packets. Based on our test cases we’re realistically saving 12 – 15 HTTP requests per page. I know we’ll see a big pay off when we start effectively caching and gzipping this stuff, but I have to admit I’m disappointed with this first round of results.

Step 1.1: Learning from Mistakes

Ok, now I understand why the RODP logo was initially flagged as not to be included in a sprite due to being a jpg. I need to roll back the Logos sprite. I also need to alter the way we’re doing the random headers. We’ve been using something based on Dan Benjamin’s method from A List Apart but in terms of site performance it’s one of our slowest resources.


We’re starting to see some minor pay off on the repeat viewing. Start render time on the current students page is now under 1 second. That’s pretty awesome, but we’ve still got work to do.

Step 2: GZip

I followed this guide as well as some official documentation from Microsoft. This online gzip test says we’re still not using compression but my web developer tool bar begs to differ.


I just alphabetized the properties in each CSS declaration, primarily as a way to clean up my CSS and make it easier to manage. But Google’s Page Speed documentation points out the possibility of alphabetized CSS seeing more efficient gzip compression. Checking the alphabetized, minified, and gzipped version against the just minified and gzipped version with the Web Developer Toolbar (Information —> View Document Size) shows 8k either way. It doesn’t break it down into units any smaller than that. So maybe it saved me a few bytes, but not a kilobyte. So if we saw any gzip efficiency change from alphabetizing the CSS it’s less than 12.5%. If we assume rounding, then it’s less than 6.25%.

Step 3: Browser Cache

This one will be labor intensive. Before I put in the hours to set expiration dates and last modified dates for our various static resources I want to make sure they are properly optimized. We’ve covered CSS and JavaScript, but all the images that didn’t find their way into a sprite will need to be tested. I started this entry on Monday morning. It is now 1:30 on Wednesday. I this step will probably keep me busy the rest of today and a good chunk of tomorrow.

Ok, the clean up and optimization process has saved me about 10,000,000 bytes and 400 files. Gzip has been running for a full day and server’s resources don’t seem to be at all taxed. So far so good. Time to dive into Leverage Browser Caching.

I have a problem in that IIS reports the wrong last-modified date for certain files. Images seem fine. PHP files and other plain text files are not. Maybe I’m missing something in Dreamweaver. Our server also sends E-Tag hashes, which are newer than and in theory superior to last-modified headers. But they are also redundant together. E-Tags are apparently frowned upon when using multiple servers, such as a Content Delivery Network. All our content comes from just the one server so I don’t think it’s an issue with us. At worst we’s sending a single redundant HTTP response header.

So half the browser caching equation was essentially taken care of for us by IIS’s default settings. The other end of that equation is content expiration. I did not like that IIS just gave me a single check box to turn content expiration on. I wanted to be able to set it up by file extension. No such luck. I turned it on and did some testing with the Live HTTP Headers extension for Firefox. I was able to confirm that our php pages were not caching. Which is good. The last thing I want is someone to spend the next 6 months with an unchanging copy of our home page stuck in their browser cache. CSS and JavaScript files were caching, along with images. I already use version numbering on CSS, JavaScript, and image sprites. For the most part other images will have specific names anyway, so that’s good. Then I tested PDF files and did not like what I found.

I downloaded the PDF of our campus map and saw that the response headers told the browser to cache it for the next 6 months as well. We may need to update the map in the next 6 months. Maybe an office location changes to a new building or something. Worse still, many of our PDFs are newsletters. Monthly newsletters. Some offices have an archive listing years’ worth of issues while others just link to the most recent issue. In the latter case, I just save the new issue over the old issue on the server. With browsers caching these files, that wouldn’t work anymore. I’d have to give each issue a distinct name and adjust the HTML links to match, greatly complicating that process.

I rolled back those changes and tried setting up expiration on a directory by directory basis. So far I’ve only done /images/, /css/, and /js/. That covers the vast majority of our static files. Occasionally a directory off of root will have its own images directory. For now I’m leaving those alone. The handful of child directories inside /images/ also apparently did not did inherit the expiration settings of their parent folder.

The downside to all this experimenting is I have to bounce the IIS service for changes to take effect. That only takes about 8 seconds by my count, but I’t to think of someone submitting their final form for online orientation during that tiny window.

Page Speed still says we fail the browser cache check, but that’s due to the FEMA widget currently on our site. All resources being served from pass. So let’s run another batch of tests.



This has been less a blog entry and more 4 days of notes to myself. Tomorrow I will try to recap the high points and present my data analysis.

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