I have a long standing IPv6 issue that I found some time to research again today. I use a free IPv6 tunnel service from Hurricane Electric which works great in most respects, except that I can't get sustained throughput for any lengthy data transfer. I've only tested it with NNTP and HTTP traffic but I assume it happens with other types as well. Since I don't depend on IPv6 for anything, solving the problem is a low priority, but I do work on it once in a while. Surprisingly, despite the slow pace, I am making progress.
Previous research has convinced me that it is an MTU issue. However, forcing the the MTU to a known value, had no effect at all. Today's research gave me a few new clues to the problem, but I just ran out of time to convert the new information into an experiment. While it was tempting to just keep hacking away at the problem, that could easily lead to several hours more work before you know it. It's always best to keep the low priority projects under control, otherwise the important stuff won't get done.