I can testify that I've used the traffic shaping features of Zentyal and have seen noticeable performance enhancements.
I have a Magic Jack Plus, which is a cheap phone service that works through my Internet connection. I created a network object in Zentyal that indicates the IP address of this magic jack device on my internal network. I've added rules to give this device priority over other network traffic.
Previously, when I was doing a lot of uploading and downloading, people would complain that I sounded like a robot in our telephone conversations. But after implementing the QOS traffic shaping in Zentyal, I haven't had anyone complain about the quality of our phone conversations.
The way I have my network set up, is that I give priority 1 to my magic jack plus device. Priority 2 goes to DNS traffic in general, and priority 3 to port 80 web traffic in general. Implicitly, all other traffic has less priority than these explicitly set rules. For me everything is very snappy.
I also have dns forwarders, which indirectly decrease web page load times. For this see my comments here:
http://forum.zentyal.org/index.php/topic,89.0.html