I’m not the developer, but I can try to answer your questions
1) performance will depend on the queries you use. With relationship field I would look into 2 way relationship. There is code on this site that will help as well as plugins available. There are a lot of other things that can be looked at but far too many to list them all here.
2) the Pro version of WPAllImport will import ACF fields http://www.wpallimport.com/
3) No. ACF uses the standard jQuery date field. To use a different type of calendar you would probably be looking at building a new field type add on plugin that uses a modified version of the datepicker like this one https://www.npmjs.com/package/persian-datepicker
