This is an easy fix. I haven't used it yet but should work in theory. You will need to set the loadavg_forum setting in the smf_settings table to the value you want.
Next open Load.php in the sources folder
And find the reloadSettings() function
Find This code
if (!empty($modSettings['loadavg_forum']) && !empty($modSettings['load_average']) && $modSettings['load_average'] >= $modSettings['loadavg_forum'])
Replace the line
And that's it. Now if your board starts to get busy now redirects it to the lofiversion and allows your server to handle the traffic better.