Usually you will be playing games on SC2 bnet using UDP since the SC2 UDP protocol is faster, incredibly tolerant to packet loss (I tested 75% packetloss and still didn't get dropped) and doesn't suffer from the ack issues or slow start issues of TCP. However, if something is preventing you from using UDP such as a firewall, router or other misconfiguration, bnet will revert to using TCP without telling you which could result in increased lag and drops.
The point of this blog post is a tool I made called SC2NetInfo that will tell you whether you are using TCP or UDP when playing SC2. Grab Sc2NetInfo.7z and extract both files somewhere. Run SC2 and start a Multiplayer Custom Game vs an AI - while this tool does work in regular ladder games, there is a small risk of crashing SC2 when it is run due to race conditions. Run Loader.exe to load the analyzer. You will see a window with the number of TCP and UDP packets sent, whichever number is increasing is the protocol your SC2 is using. Requires VC 2008 runtimes if you don't have them already.
If you are using TCP, this means that bnet wasn't able to communicate over UDP for some reason. Check any firewall software to ensure SC2 isn't blocked or anything in your router (you don't need any port forwarding for SC2 - turn it off if you have any). SC2 UDP runs on port 1119.
WARNING: While this doesn't modify SC2 at all, this is still a 3rd party program and you should use this at your own risk.