Save map in a file at runtime freeze the engine

Because my game will have an level editor I need to be able to save and load maps created with it, because I don’t know every thing about the engine I created my own map save system using Json but when I load a map it freeze the whole engine depending of the number of elements present in it ( it’s logic ).
So I wanted to know if there is a better way to do this or if I can add a loading screen while the map is loading.

Here is my blueprint for loading and saving:

Place objects on the stage in the editor. And at loading change their properties according to the saved data. Spawn gives an additional unjustified load.