Great companies will always be looking for ways to streamline their hiring process and improve the candidate experience. You can be honest with the interviewer on your experience; however, keep your reply positive.

If you have found the process to be slow and frustrating, you could express that by saying "I have experienced faster candidate screening processes in the past but I am sure you are doing your best with the amount of applications you receive."

This would be a much better response than saying "Your process is very slow so I would work on fixing that."