Windows 7 64 bit, core 2 duo, 8 gb ram, plenty of open space on the hard drive.
Files work just fine in CS5 and CS5.5. Opening one of them in CS6 and working in them when just opened is also fine. After saving the file as a CS6 document, closing, and reopening in CS6, selecting text becomes extremely slow...merely putting the cursor in a text frame results in significant delay--the toolbar goes blank and takes several seconds before it redraws. Proceeding to select text and typing changes causes similar behavior.
Opening an IDML file causes similar behavior after the file is saved as CS6 and closed.
Files created in CS6 do not have this problem. It only seems to happen to files converted from previous versions or from IDML.
Does not happen consistently, so I suppose it could be a font issue, although the files in question were originally built on a Mac and are being opened on the PC.