Reading/Multimedia/Media Viewer/Speed reports

We have gotten lots of reports that it can take very long for MediaViewer to display (unblur) the images (some people mentioned 20-30 second waiting times); our tests and metrics, on the other hand, show a one second median loading time (slightly faster than the old method of opening the file page). We need to find out under what conditions does the application become so slow; if you experience something like that, or know other people who have experienced it, please help us collect more information.

Ideally, we would like to know the following:
 * how long did it take for the image to load? (time spent between clicking on a thumbnail, and the blurry image becoming sharp)
 * can you reproduce the slow loading time with other images? (Use images from a different wiki page, so that they are not preloaded)
 * can you reproduce the slow loading time with same image, after refreshing the page with a clear cache (Ctrl-F5 on most browsers)?
 * what OS/browser do you use?
 * what kind of machine do you use? (Especially if it is an older/slower one, pointers on CPU speed and memory would be helpful.)
 * what kind of internet connection do you use, what bandwidth does it have?

If you are comfortable with using the network tab on the web console of your browser (F12 on most browsers), and can look at it and tell us in detail which requests took up the majority of the loading time, that would be even more helpful.

Initial loading time of the javascript itself
I'm generally most affected the first time each session I click on an image - the screen turns black, nothing happens (for at least 5 seconds) and clicking "back" pushes me two pages back. When I saw the existence of this page, I tried replicating the issue but was unable to. I'm thinking if I cleared my cache the issue would arise with the first new image click, once again, as first the Media Viewer script needs to transfer to me, then my javascript engine needs to understand it, then it has to actually run. ([]) 16:06, 27 January 2016
 * Thanks for the report! The first time you open MediaViewer (after a cache clear or after not doing it for several weeks) is indeed somewhat slower since the JS files have to be downloaded. Normally this should be a small delay (the script is about 160K if your browser supports compression, which all modern browsers do; a typical image is going to be about the same size). But it's always possible something goes wrong on a specific browser or a specific connection; can you share more details about those? --Tgr (WMF) (talk) 07:26, 28 January 2016 (UTC)
 * I experience it both at work, using Windows 7 with Chrome browser (40.something) and at home, using Linux (Ubuntu 14.04) with Firefox (whatever's current). I'd say it's definitely worse at home, but then again, my own computer's significantly older. Both locations have a 100Mbit connection, so the size of the file should have negligible impact - hence my speculation about ping/roundtrip time and javascript loading/running. The worst impact is that "back" kicks me back from the original page (e.g. Going from Napoleon to Code civil, clicking an image there and then clicking back will kick me back to Napoleon rather than Code civil) if I click back before the image has started to load. Which I tend to do, as my screen (the browser window) goes black for a couple of seconds. After that, I generally open the image in a new window, keep reading the article, and when I see the tab's loaded, I'm ready to roll. ([]) 15:31, 28 January 2016