I think the point in your case is that your slow downs or freezing aren't likely bugs. If lowering the buffer enough prevents it, then that's just a matter of memory usage caused by so many windows. Yes, it may be possible to optimize things some more to make it possible to handle larger numbers of windows with large buffers, but in the end, you're still going to be limited to what the system can handle. That's not a bug. The same with font linking - it can possible be optimized some, if that's even necessary when we get the new unicode release, but it isn't a bug.
It's no different than if you tried to open a hundred programs... unless they were really small, that many would likely slow Windows (or any OS) to a crawl. That isn't the OS's fault - it's not a bug. Just a limitation on system resources.
As such, I think it would be good to stick to actual bugs, or what are possibly bugs, in this thread/forum to avoid confusing the issue and making it more difficult to find the actual bugs. If you'd like to request optimization of mIRC's features to allow so many windows, that is better placed in the feature suggestion forum even though it's not technically a feature. So many people with completely different (or seemingly different) problems have used this same thread just because the results are the same... it makes it difficult to determine what the cause is or even if they are related.