Go to Display > Heads Up Display > Polycount. How many Tris/Faces do you have? Anything over a few hundred-thousand is going to begin to slow you down.
How do I do that and what will it means for the object (i.e. will part of it dissapear, will they behave differently etc.)?
depends on what the imported objects where, if you've imported millions of polys or particles or even instanced paint effects this is going to cause some slowdown.
Try removing the history from the imported objects.