Maps in item frames load significantly slow until/unless copies of these maps are inserted in the inventory.

A discovered map in an item frame loads significantly slow when opening a world unless a copy of that map is inserted in the inventory. While the copy is in the inventory, the map in the item frame loads fast as always. If the copy is removed from the inventory the loading speed of the map in item frame is slowed again.

This video can describe it better:
http://youtu.be/JwPaafsaiYo