We completed our migration to Team Calendars last weekend.
It took nearly 15 hours of manual work to import that calendars, set permissions on them, and update the wiki markup on pages where the calendar was used.
Much of that time was spent waiting.
If you're importing a large number of calendars the performance of the "My Calendars" section gets slower for the following actions as the number of calendars listed increases:

1. Importing an additional calendar
2. Setting permissions on a calendar
3. Removing a calendar from the list

As a Confluence administrator, you may wish to have a large number of calendars visible on your "My Calendars" page. However, the performance impact is huge.
From what I can tell the performance slow down is a combination of re-loading all calendar information for changes that affect only a single calendar, and the browser then parsing the returned calendar data, generating the DOM, and re-painting.