I have figured out the problem, but am waiting for a permanent solution from ESRI support. It appears that ArcGIS 10.2.1 introduced a new option: Editor > Options > Versioning > Automatically save changes after each edit which is checked by default and must be manually unchecked each time you open a map - unchecking the option works for the current map/session, but the next time you open a new or existing map it's checked again. The slow performance is caused by the editor writing to the database and committing and saving each attribute/feature edit to the database one at a time after each edit - normally you only do this once when you save changes at the end of each edit session.
FYI - this issue has been logged by ESRI as a 10.2.1 bug:
[#NIM098991 The option to "Automatically save changes after each edit" within the "Editing Options" does not stay unchecked after the Map Document is saved]. For updates on this issue, you can go to ESRI's Customer Care Portal site.
At this point, there is no workaround for this issue. You will have to ensure that your users uncheck the option to "Automatically save changes after each edit" each time they open the Map Document.