Cleaning with PU command is mainly because some blocks occupy a large "space" and can be cleaned selectively. Close it if there is a DWG reference.

It is unscientific to continue to draw in a document. Files are getting bigger and bigger, occupying more and more memory, running slower and slower, and it is easy to produce inexplicable errors. But also easy to cause file damage and so on.

It depends on how big the file is and how much it contains. If it's really big and the computer configuration is not too high, it's really slow. Every time you refresh and move the viewport, all the graphics will be involved. If there are too many graphics, it is normal to be slow. There is really no other good way.