Yep, same problem here.
Lots off CPU usage for the message plugin.
Site almost unreachable.
1. loaded mysql high
2. blocked user browser.
Seems that it could be the same problem. I feel it is somehow related with mailbox upgrade to messages.
More on that in my post: http://www.oxwall.org/forum/topic/21636
I had the same issue. The problem is with the offline list. If you have many friends and on your chat window the offline users show up then you got a problem. If you see in the Messages plugin settings it also says its a bad idea to have the offline users list enabled for big sites (I think the issue is when you have more than about 200 friends).
Guys, mailbox plugin has been updated, please update it. The notification should appear in your admin panel within next 24 hours.