I was experiencing the same type of issue (adding to cart was slooooow...) an after disabling EAV cache it’s way faster.
If someone have an explanation about this it would be much appreciated.
I am having the same problem with add to cart. I have noticed that if I go into magento connect and reinstall the core files, it improves for about 24 hours and then i have to do it again. I tried messing with the EAV cache and that didn’t do anything really.