Just tell them what you've said here, although you might want to leave out the bit about things being a farce. Say you're looking forward to working with your new employers and that one of the things you're hoping for is the implementation of a proper appraisal system and improved communications.
If they ask why, then go on to say that whilst you don't want to sound as though you're 'slagging them off', you've found it difficult to gauge your own performance levels over the past five years because these systems haven't been in place (or at least not actively).
The interviewer won't be there to apportion any blame. Personnel will have been asked to chat to you all in order to get an overall view of the workforce and any issues that affect it. From there they can identify weaknesses and hopefully take steps to put them right.