Personally I think this is the worst kind of management. To let someone go based on performance when they have had no indication of any kind and / or the opportunity to improve is extremely poor.

You should have been given regular performance appraisals where these issues should have been raised and you given guidance on improvement.

Whilst I am sure you are upset to be leaving it doesnt sound as though career development is high on the agenda so you may well be better off out of it in the long run.