Really drastic way:
Use a very lightweight mobile style which doesn't show avatars, signatures and almost all images as the default style for all users.
(we used a 58k style when we had our spam-friday, a day where we removed the posting limitations for 10minutes and encouraged our users to give it a shot, once we grew bigger it crashed our server everytime, so we had to stop doing that

)
Anyway for vb4:
https://vborg.vbsupport.ru/showthread.php?t=249277
Idea is to reduce the http requests (pictures and javascript) and send less data, so even under massive server load the forum would still manage to load in a reasonable time.
Another way to reduce http requests is our sprites addon:
https://vborg.vbsupport.ru/showthread.php?t=266696
Cheers