Motivation
AdvancedSearch should not slow down the loading of the Special:Search page.
The AdvancedSearch form can only be interacted with, if you expand it, and if people are not using AdvancedSearch it is not needed at all. Hence it should be loaded last, but ideally, before the person actually wants to use it.

Task

• When Special:Search is loaded, the (not visible) advancedSearch form does not load until 500 ms after reaching the page
• Do some performance checking how this behaves on slower internet connections

Notes

• @Catrope extended @thiemowmde 's patch, which now needs a merge: https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/AdvancedSearch/+/446837/
• Step 2 could be a proactive lazy loading of the contents of the AdvancedSearch form, if that is still necessary
• In testing, 500ms turned out to be a good value for delay. It will then take another 100ms usually to create the form, which should still be fast enough for the user not noticing any delays when opening the advancedSearch form