I'm seeing the same behavior in Word 2013 running on Windows 7 on a Lenovo T61p laptop. The file is a 177kb .docx file that contains both Tracked Changes and Comments. The slowdown appears to occur as soon as I open the Reviewing Pane or add a comment. The behavior consists of repeated frequent spinning circle machine-busy indications and grayed-out file-not-responding indicators. The only way to fix the situation is to close the file and Word down and reopen. But the behavior reoccurs as soon as I either open the Reviewing Pane or add a comment.

Best,
Alan