I am finding the same problem with CS6 that was present in CS5 on my computers (PC, Windows 7, 64-bit, 16 gig ram): when I have more than 40 pages of text with multiple text boxes ( for running annotations), the screen rewrite gets slower and slower. I have a document of 100 pages that takes about 2 or 3 second to input each letter!
I have read all the suggestions about resetting preferences, changing display performance, etc. etc., but nothing helps. I even reformatted the entire file from scratch to make sure there were no errant text frames. Same s-l-o-w results. When I put the same data in a CS3 file, it ran perfectly and at top speed.
When I contacted the Forum at the time of a trial of CS5, a problem was acknowledged and it was suggested that this will be addressed in CS6. This is truly frustrating.
Can you check in your InDesign Preferences (CTRL or CMD K) under Interface and change Live Screen Drawing to "Delayed" instead of immediate.
I too get slow text files in InDesign - particularly using Tables that span multiple pages and also with using the feature of Span/Split Columns to span text across two or more columns.
Seems the more of these I have the slower the file goes.