Thank you for reaching out to us in the Community. I'm sorry the site appears to freeze up while in use. Is it when you're on a certain tab (Campaigns, Contacts, etc.) or when performing a certain function (editing the e-mail, uploading a list of contacts)?
Have you tried another browser? A lot of the time it's something about the browser that is intefering with the site. Or, you could try Safe Mode/Incognito in your regular browser and see if the site functions normally then. It's possible there is a browser extension enabled that is not compatible with our site. This FAQ will show you how to access Safe Mode/Incognito.
Also make sure to try an Internet speed test to see if you're running at least 2 mbps upload/download.
Let's see if trying another browser or using Incognito helps. If it doesn't then please reach out to us at social_support(at)constantcontact(dot)com with a reference to this post and we will troubleshoot further to figure out what is going on.