When choosing a property there is a single, pretty simple "wbgetentities" API call triggered. At the moment I don't see a reason why this should cause a "substantial" delay other than what an API call usually causes.

The previous autocomplete call ("wbsearchentities") doesn't include the required information. Example response: {"id":"P46","url":"http://localhost/repowiki/index.php/Property:P46","label":"Parent"}. I don't think it's a good idea to change this.

A possible solution would be to use the fetchedEntities local store to avoid these API calls if the property is already in the store. Currently the API is always requested and the store is never used (but the API calls result is put in the store). Not sure if this is a good idea. When will an outdated property in the store be invalidated?

Another option might be to just display a simple text field immediately, and only add the various additional datatype-dependent stuff after the datatype retrieval is finished.

(In reply to Yair Rand from comment #2)

display a simple text field immediately

Good idea, I will keep that in mind. Another idea is to put the data needed into data values in the DOM and use that instead of querying the API. But this depends on how we fix bug 54098.