I will join those reporting a severe performance issue with v1426. The symptoms are that everything runs about 100 times as slow, making the system appear to be locked up, nearly unresponsive to input... but if you are infinitely patient, everything will eventually happen. I've seen the exact same thing occur in the past with one of those 'other' antivirus programs. My solution so far has been to revert to v1407.
Obviously, not everyone is experiencing this symptom. In fact, it is happening for me on only one of three computers. The other two are running V1426 with no problem. Why is this? I don't know.