When a large document is opened the cms response time drops. When editing and entering text this is very noticeable as text may take almost a second to catch up to typing. Other actions are affected as well. The lag occurs when the document is opened for viewing not just when editing.
A large document is one have multiple content block items, and the content block items in this case can be quite complex in and of itself.
This behavior is only evident in Chrome. FF and IE behave correctly.