Ok, so you have computer A and B. You set up a schedule to set "when" the restriction applies, Website filter to set "what" is restricted, and used Access control to bind the two together and assign the rule to "who" (mac address) is restricted. Good in theory. There are some limitations though.
1. It blocks website by denying any DNS request for those specific domains. If your computer already has DNS cache for those sites, or host files, the block will only block additional content off that page. For instance. You block CNN.com. But before you block it, computer A has already been to the website, so has a cache of the domain resolutions. So it knows the IP of cnn.com. It goes there, reads the directory, and starts loading the content. Some of that content will already be on your computer, say picture 1 and 2. But say Picture 3 has a different file name on a different server that isn't in your computers DNS cache, it has to do DNS query, and that is where the router blocks it. That could explain the load, then slow crawl to a stop. It knows the IP, it loads what it can from the website locally, requests more and waits until the query times outs. But isn't that what you want it to do? Block that site?
2.It only blocks queries made through the router to the DNS the router obtains by DNS, or the one set static to it. If your computer has it's DNS set static, it totally bypasses the restriction.
3. You can't specify sub-domains, at-least with this tech. You could open up a OpenDNS account and specify your block, or if you really want to fancy dan it, set up your own DNS.