Avast Free 5.0.418 Persistent Caching

I wanted to know if there was any way bad stuff could get past in the Persistent Caching option is turned on? I feel that Transient Caching is fine, as a reboot or virus definition update will scan all the files again. However, with Persistent Caching, these files will never be scanned again. How safe is that really?

Also, if I turn Persistent Caching off, how much slower will the system get, and how much extra protection will I start receiving straight away?

Same question with turning Transient Caching off also?

Files that have information stored in persistent cache will be scanned again if they change. I don’t think you’ll gain any extra protection at all by turning it off.