Had this problem for a while now, however, I have managed to solve it so thought I would share it. Not sure if it is a common issue and I currently use AutoCAD 2012.
As the thread title suggests, sometimes after exiting the block editor, my drawing begins to 'lag'. It becomes very choppy when panning or using the mousewheel zoom and commands take a little longer than usual to respond. For me, all I have to do is a zoom extents and it's fixed, no idea how or why.
Hope this may help others in the future