Wikimedia Bugzilla is closed!

Simply test it: During the whole upload process 2 intervalls *per file* are not cleared:

var se = window.setInterval;
window.setInterval = function (fn, t) {

return se(function() {
console.log(fn, t);
fn();
}, t)
};

If you upload 50 files, this means 100 events fire (all the time) within 500ms, executing a function that is computing some numbers from the DOM (object position) and setting CSS. Not good for old machines and slow browsers.

Culprit:
/extensions/UploadWizard/resources/mw.UploadWizardUploadInterface.js

moveFileInputToCover: function( selector ) {
//...

this.moveFileInputInterval = window.setInterval(function() {
update();
}, 500); 
}