Since mediawiki.org finally moved to '21, I was able to run the performance tests against production again. We now have a figure for how bad the cold cache experience can be. On an average-sized browser window, the cold cache image load in media viewer takes around 200% of the time it takes for the image to appear on the File: page with a warm cache*. And it takes around 230% of the time for a large browser window. We're talking seconds of difference, that's a long time to wait.

When we first built mmv.bootstrap (the on-demand loading of JS), we didn't have the click catcher (mmv.head). Now that we do, I think we can afford to sacrifice some user bandwidth and just load Media Viewer's JS with the async attribute (which means it wouldn't block page rendering). And mmv.head would take care of catching and replaying clicks that happened before Media Viewer's JS was loaded.

Doing that means we could get rid of mmv.bootstrap entirely, increase the cold cache performance (chances are, the thumbnails on a given page will take longer to load than Media Viewer's JS and CSS) and simplify our code.

Any objections to doing this?