IIRC it’s channel because it’s a channel specific command (PRIVMSG), Things like JOIN or PART would be global commands but I could be wrong.
Edit: I just tested 5 different channels 50 messages each at the same time (Mod in 2 of the channels) and all messages went through fine with the connection staying open still able to send messages after
Edit Again: My test was flawed do to poor pre-coffee code see @3ventic’s reply under this for a real answer.