I honestly can not fathom how you break the game so hard. How can a Paladins channel spells make everyone in the raid lagg?
Simple: There's a bug in this build that causes some cast-time paladin spells to have bad (negative) heal prediction values, which the UI that displays predicted healing isn't expecting, causing a ton of client lag for everyone that is trying to display those values. This is fixed in the next build. Turning off predictive healing in the UI may help, in the meantime.
Every new patch involves a tremendous number of changes to the game -- both obvious new content like adding new dungeons or outdoor areas and countless behind-the-scenes changes to code, client/server interactions, and so forth. With thousands of such changes, there are unfortunately going to be some bugs, and often some very unexpected ones. Part of the purpose of PTR is to catch as many issues like these as possible, so that they don't make it to the live servers and impact players' experience there.
Thank you, as always, for your patience and for helping test the 5.2 patch.