Please follow the below steps.
Go to Preferences.
Click on the General tab,
Uncheck the Option " Show Add Photo Screen"
Restart the Lightroom.
Also, let us know the Operating system & version you are using.
I have the exact same problem, constant freezing every few seconds, and unchecking "Show Add Photo Screen" totally fixes it! What a horrible bug! I'm not even importing anything. Lightroom is completely unusable with this checked.
Windows 10, i7 2600K, 8GB RAM, 250GB SSD, R9 video in case Adobe wants to get around to fixing it.
Upgrade to Lr6.3/2015.3. The old import module has returned so you don't have to deal with "Show Add Photo Screen".