Sometimes I notice that my dev environment is starts responding slowly, like 10× slower. When you notice this, try firing up vagrant ssh and try something like wp cache set food bard. If you get an error message about it not being able to be set, Memcached may be on the rejecting cache additions (for an unknown reason), and so the Object Cache is no longer able to store data intended to persist across requests.
To fix this issue, just do sudo service memcached restart. You should then be able to do wp cache set food bard and see this:
Success: Set object ‘food’ in group ”.
I’ve encountered this problem on VVV so it’s not unique to Quickstart.