We currently call this.lazyMenuCreation() on the toggle event. Depending on various factors, there can be a perceptible delay before the menu appears. On approach to get around this is to use setTimeout() on the model initialization and build the menu as part of the page load process; the toggle on the menu is then very fast.
Customize query in gerrit
|mediawiki/core||master||+2 -0||RC Filters: Lazy build menu on model initialization|