"I've noticed when you first join a channel and double click on it (bringing up the /channel dialog), all text seems to halt from echo-ing to the channel until that dialog is closed, at which point, all text that was sent while the dialog was open will echo to the channel at once.

mIRC echo's timestamps based on the time of the local machine, so yeah, the timestamps show it's all happening at once (give or take a millisecond or two). Is it just my client that's doing this or can others also verify?