The most resource intensive is initial page processing. After that Midnight Lizard does not consume resources if page is static. When you open pages one by one in most cases it is not that difficult for Midnight Lizard to process them. But when you already have a lot of opened tabs/windows and then you turn Midnight Lizard on - it starts processing all of them at the same time. Some browsers take care of this problem better than others. And I will try to fix it as well.
With notification about slowing down you can fight with Midnight Lizard's simplified mode. In the next release I will try to optimize default auto mode or make simplified a default one.