Reading/Web/Lazy loading of images on all wikis

As of Thursday, Aug 18, 00:05 AM UTC images on the mobile site of all Wikimedia projects are lazy loaded. The purpose of this page is to measure the impact this had on our users. In particular we were curious about English, Indonesian and Italian Wikipedia's.

Impact on bytes served to users
Comparison of sampled Wikipedia mobile web image and "core bytes" (essential HTML, JavaScript, and CSS) data transfer and pageviews 22-28 August 2016 (lazy loaded images generally on) against 8-14 August 2016 (lazy loading images only enabled on a relatively small set of wikis) suggested that lazy loading images saved approximately 43.63% in image data transfer, contributing to a 14.48% savings in the sum of image and core bytes.

Projected to 52 weeks, this suggests that some 450,000 GB of data transfer would be saved annually on mobile web Wikipedia due to image lazy loading.

The queries, BASH scripts, and resultant data are available as a zip file. Go to the region near cell P5690 in the ODS file for aggregate calculations.

Method for individual wikis
Our previous experiments showed that we prevented the shipping of unnecessary bytes to our users. To measure this, using Hive, we analyzed request logs 10 days preceding and following the change. Since the change happened on the 18th August, we looked at requests from the 8th onwards.

Given the time it takes to analyse the amount of data we are crunching here, a query for each day was run.

The query that was run looked at every request in our access log made to our image server where the referrer of that image request was a page that matched a URL of a page in the main namespace for that wiki: The above query was repeated for English Wikipedia.

Bytes savings on Indonesian Wikipedia
Day of change is in bold.

Bytes savings on English Wikipedia
Day of change is in bold.

Method
To measure impact we took a look at data we had collected via real users via the NavigationTiming API over a 20 day period (10 days before and after the change) were taken of 3 wikis using the following query:

Results
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Impact on fully loaded time on English Wikipedia

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