Page Weight Matters

Three years ago, while I was a web developer at YouTube, one of the senior engineers began a rant about the page weight of the video watch page being far too large. The page had ballooned to as high as 1.2MB and dozens of requests. This engineer openly vented that “if they can write an entire Quake clone in under 100KB, we have no excuse for this!” Given that I agreed with him and I was excited to find a new project, I decided to champion the cause of getting the YouTube watch page to weigh in under 100KB. On the shuttle home from San Bruno that night, I coded up a prototype. I decided to limit the functionality to just a basic masthead, the video player, five related videos, a sharing button, a flagging tool, and ten comments loaded in via AJAX. I code-named the project “Feather”.

Even with such a limited set of features, the page was weighing in at 250KB. I dug into the code and realized that our optimization tools (i.e. Closure compilation) were unable to exclude code that was never actually used in the page itself (which would be an unfair expectation of any tool under the circumstances). The only way to reduce the code further was to optimize by hand the CSS, Javascript, and image sprites myself. After three painstaking days, I had arrived at a much leaner solution. It still was not under 100KB though. Having just finished writing the HTML5 video player, I decided to plug it in instead of the far heavier Flash player. Bam! 98KB and only 14 requests. I threaded the code with some basic monitoring and launched an opt-in to a fraction of our traffic.

After a week of data collection, the numbers came back… and they were baffling. The average aggregate page latency under Feather had actually INCREASED. I had decreased the total page weight and number of requests to a tenth of what they were previously and somehow the numbers were showing that it was taking LONGER for videos to load on Feather. This could not be possible. Digging through the numbers more and after browser testing repeatedly, nothing made sense. I was just about to give up on the project, with my world view completely shattered, when my colleague discovered the answer: geography.

When we plotted the data geographically and compared it to our total numbers broken out by region, there was a disproportionate increase in traffic from places like Southeast Asia, South America, Africa, and even remote regions of Siberia. Further investigation revealed that, in these places, the average page load time under Feather was over TWO MINUTES! This meant that a regular video page, at over a megabyte, was taking more than TWENTY MINUTES to load! This was the penalty incurred before the video stream even had a chance to show the first frame. Correspondingly, entire populations of people simply could not use YouTube because it took too long to see anything. Under Feather, despite it taking over two minutes to get to the first frame of video, watching a video actually became a real possibility. Over the week, word of Feather had spread in these areas and our numbers were completely skewed as a result. Large numbers of people who were previously unable to use YouTube before were suddenly able to.

Through Feather, I learned a valuable lesson about the state of the Internet throughout the rest of the world. Many of us are fortunate to live in high bandwidth regions, but there are still large portions of the world that do not. By keeping your client side code small and lightweight, you can literally open your product up to new markets.

Update 2016-04-06:
At imgix, we’ve built a tool to better monitor your page weight and recommend improvements. We call it the imgix Page Weight Tool.


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