I'd rather know that someone is lagging and atleast be able to be aware that I may need to perhaps, close an extra program, or remove some projectiles or something.
It's better than having a silent person lagging, then decide to up and leave without even telling me why. I like to think that I can help people in my server.
And yes, I get annoyed when people are overly shouting "LAG!!!!" or similar stuff. It's much politer to just say "Sorry, I'm lagging some, over here".
Just a nice little message, rather than a torrent of "LAGLAGLAGLAGLAG" washing over the host's screen once the servermembers stop lagging and their message comes through.