Thanks for your question.
RadCalendar is a heavier control with lots of visuals and complex logic behind. Generally RadWindow will wait for the hosted control to load and then open itself with an animation. You may try to remove Window's animations which will improve the performance a bit but still, it will not be instant. I think a better solution in your case is to have the calendar on a separate page, not displaying it on the current one which have other visuals on it, increasing the overall fill rate and performance.
Most of the Calendar's loading time is taken by the creation of the grid cells - ContentControl instances. If we use ContentPresenter instances then it would be faster but we will lose the styling capabilities of each cell. We will perform some additional measurements to see whether something may be improved.
I hope this information is useful. Let me know if I can assist you in some other way.
the Telerik team
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