[DatePicker] opening the Popup takes some time

[svetq]

Describe the bug The opening of the DatePicker or DateTimePicker popup takes some noticeable time.

The issue is caused as the Calendar is initially rendered invisibly to make some required calculations before it is rendered a second time visibly at the proper location.

To Reproduce That can be observed in the following example:

https://stackblitz.com/edit/angular-lwkkcw?file=app%2Fapp.component.ts

[svetq]

additional details provided in private support thread 1555202

[mbechev]

Also reported in private support thread: 1558714