Quiz: Guess the Impact of 50 KBytes on Page Load via DSL
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As you might recall, www.alice-dsl.de is one of our web properties we’re responsible for. And due to this we had a rather busy week. Reason for this can be found here. But I do not want to comment on that, but rather about the technical outcome Because due to “the story” we had to redo quite a few of our graphics on our website.
And THAT was actually one thing I was eagerly waiting for.
Before “the story” we had a page header with a rather difficult image. Our current design language pretty often challenges us (or better, our agencies) quite a bit, as the graphics often consist of a photo-realistic part, which is in front of a colour-gradient background. So the different compression methods fail one way or the other. If we compress using PNG8, the quality of the photo-realistic part degrades rather badly. If we compress using JPG, the colour gradients and sharp edges become really ugly, making it necessary to compress with high quality settings, resulting in rather large files. If we would work with 2 files using transparancy and different compression methods, well, we would have 2 HTTP Requests instead of 1.
So, to make a long story short, this header image was formerly 72 KByte of size. I asked my colleagues to make sure, that the new one would be much smaller in size, and to really push for that. What we got back was an 18 KByte image. I wasn’t totally satisfied with the result, as the agency used JPG again, even though the new image wasn’t really suitable for JPG. With PNG8 I was able to further reduce it to 8 KByte instead of 18 KByte. Nevertheless I was happy enough by the reduction of ~50 KByte.
Now back to the quiz question: What do you think, how much faster our site reaches its time to visually complete due to that reduction? (using the Frankfurt node of WPT @ 1.5 MBit/s, 50 ms RTT and IE8)
The answer might surprise some people (at least it did within our company). Normally you might do a napkin calculation like this: 50 KByte = 400 KBit. 400 KBit on a 1.5 MBit/s line should be transmitted in ~1/4th of a second = 250 ms. So the page load time might decrease by 250 ms.
But this omits from the equation TCP Slow Start! If you are unfamiliar with TCP Slow Start, the VERY, VERY simplified and brief explanation is: When a TCP Connection is established, it doesn’t utilize all available bandwidth from the beginning, but instead is “slowly” increasing the bandwidth utilization, to test the available bandwidth. A TCP connection “can’t know” the available bandwidth, so in order to not overload the network, it starts slowly and increases over time.
Sooo… What was the question again? Oh, right! The benefit of the image size reduction! Getting back to our napkin calculation: The former image was ~70 KByte of size, which is roughly 0.5 MBit, which should load in ~333 ms over a 1.5 MBit/s line. Right?
Again I used WPT with its tcpdump feature and loaded the image. And the result is, without DNS resolution…: ~666 ms! So it is roughly the double! Why so? You guessed it, the reason is TCP Slow Start.
As you can see in the image above, using Wiresharks TCP Bandwidth Statistic Analysis, it takes close to 400 ms before this TCP Connection has reached its bandwidth limitation!
Now the problem is, that the header image is quite at the beginning of the HTML basepage. And therefore it gets loaded on a rather “cold” TCP Connection. With IE8 opening up to 6 connections per server, you will start close to the beginning of the page load with 5 “cold” TCP connections.
Just recently a lot of smart people started working on circumventing the limitations of TCP Slow Start in different areas. So SPDY for example multiplexes requests on a single TCP connection, therefore going through TCP Slow Start only once. Firefox now reuses connections by the highest CWND. Starting with Linux Kernel 2.6.33 the initial CWND has been increased from 3 to 10.
But, as you can’t force your visitiors to use a specific Browser, or you might not be able to choose your Linux Kernel, your best bet is still:
And don’t be fooled by the fact, that 50 KByte on a 1.5 MBit/s line sounds neglectable.
While rambling, I almost forgot the initial question: The impact of this 50 KByte saved on Time-to-visually-complete. See yourself!