Unfortunately, there isn't a guide to interpreting the various levels of log I can point you at; many of the specific diagnostic messages relate to events in the code that you need the source to interpret exactly. At the the more detailed levels what the log includes is essentially a structured dump of the network packets similar to what you would capture with Wireshark, but taken from the viewpoint of the internal state machine.
> What kind of entries am I looking for to indicate why and where this slow multicasting is happening?
The main thing that happens when multicasting slows down is that some client is losing data for some reason, and that is causing a lot of retransmission. Figuring out the exact reason why is hard; the problem is often somewhere in the network outside the server or client machines, where it's simply impossible for them to just come out and tell you what is going wrong because there is simply no indication why the data doesn't get through.
In reading these kind of logs, we are not looking directly at the root causes of faults; instead we are seeing the consequences of them. Working back from a surface error to the root cause is not a matter of looking for a specific kind of message.
So, what you're looking for is basically what anyone is looking for in a diagnostic trace of any kind; something
different to the normal case, so you can use that as a starting point to work out what's not normal about your situation. Of course you have to first learn what the normal case looks like... but that's not as hard as it sounds, since you can turn on the log any time you like and watch what it says.
The best advice I can give you is to get used to what's in the log in a case that works. Then, in a situation where it doesnt work, if you see something unfamiliar in the log, that's probably significant. The same applies to any kind of network tracing.
All that general stuff said, if you have a specific question about a specific message I can answer it (and I do my best to review logs that are sent to me).