This repository github.com://arielf/speedtests contains data about internet-speeds and code to allow anyone to reproduce the results.

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# Install prerequisites
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sudo apt-get install make coreutils perl speedtest-cli
sudo apt-get install r-base r-cran-data.table r-cran-ggplot2 r-cran-scales

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# Clone this repository, and cd to it
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git clone https://github.com/arielf/speedtests
cd speedtests

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# Generate N-row data-set for N-closest speedtest data-centers
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make data

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# Generate a chart from the data-set
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make chart

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# Today's speed-test chart should be created at:
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Data/<yourusername>.<yyyy-mm-dd>.tsv.png

• The speedtest utility (apt install speedtest-cli) collects speed data from various data-centers
• my-speedtest.many is a perl-script wrapper around speedtest. It figures out the closest N data-centers & checks the speed to each of them.
• speedtests.R is an R script to generate a chart from the data-set
• speedtests.R depends on R + the R libraries data.table, ggplot2, & scales
• speedtests.R calls nomacs to view the generated chart. YMMV. Change it to your favorite image viewer or ignore the error at the end.
• See Makefile for the full flow.
• TLDR: if you have all prereqs installed, just type make

Comcast claims my plan should give me dowload bandwitdh of 75 Mbps (Mega-bits per second).

They don't say anything about upload-speeds or ping round-trip latencies, which is a shame.

When testing my speed using Comcast own speed-test service, the results say I'm getting the promised speed and then some.

Here's an example of a screenshot from a test by Ookla (aka speedtest.net, owned by Comcast) where all numbers look good and as I expect them:

However whenever I test my speeds using other independent speed-test providers, my results are significantly and consistently worse.

How much worse?

• Average download speeds are roughly 2x worse
• Average upload speeds are roughly 2x worse
• Average ping round-trips are roughly 3x to 6x times worse

The data below, collected over different times, using various services, speaks for itself.

Raw speed-test data (TSV format)

• The darker blue points at the top-right of the chart are Comcast data-points. The very top ones correspond to speedtest.xfinity.com, the slightly lower ones, correspond to speedtest.net (Ookla, owned by Comcast).
• All independent internet speed-test providers, except one (Converse in Code) fall to the left of the center (<5.0 Mbps upload speeds) and none come close to the speedtest.xfinity.com result in either speeds or latencies.
• Interestingly, there is an additional Comcast data-center (52 km away) which is not my default speedtest target. Even though this is a Comcast data-center it is throttled-down just like the other independent providers. Seems like Comcast is throttling my speed globally, except when I directly use one of their own speedtest to check my speed.
• Seems like Comcast is being selective about which peering points or sources it is throttling-down. The default "speedtest" I get in the browser is not throttled-down, and makes Comcast apppear as if it is the fastest of all providers.
• In the past, I used to see consistent speeds between Comcast speedtest sites and other independent speedtest sites in the Bay Area. I was seeing ~90Mbps down and 5.5Mbps up pretty consistently, regardless of which speed-test service I used. More details in my Comcast experience.

By far, the fastest of all "speedtest" services for me, is one that is embedded in Comcast web site (under "check your speed") and is pointing at speedtest.xfinity.com. This one is further distributed over a CDN (multiple data-centers close to the network-edge) to make it appear even faster and less representative of the network as a whole. Here's the DNS answer showing it:

;; ANSWER SECTION
speedtest.xfinity.com.  162     IN      CNAME   cdn.speedtest-prod.xcr.comcast.net.
cdn.speedtest-prod.xcr.comcast.net. 0 IN A      69.241.75.162
cdn.speedtest-prod.xcr.comcast.net. 0 IN A      96.113.184.102
cdn.speedtest-prod.xcr.comcast.net. 0 IN A      69.241.90.114
cdn.speedtest-prod.xcr.comcast.net. 0 IN A      96.112.146.30
cdn.speedtest-prod.xcr.comcast.net. 0 IN A      96.113.184.110

In reality, the speed claimed by Comcast is not representative of real average speeds I experience.

The service levels are significantly worse than advertised, when accessing most or even all parts of the internet (other than Comcast dedicated & misleading speedtest sites)

Using the code and instructions above on Ubuntu Linux or similar, anyone can repeat my experiment. I encourage readers to download and share their results so we have a larger sample. Please share what is the speed Comcast (or any other ISP) says you're getting, vs what you actually get in this experiment.

• A similar experience by another Comcast subscriber on xfinity forums
• My own recent experience dealing with Comcast