I've noticed that on my application there is a massive performance impact when setting the "active" pseudoclass in Element::ProcessDefaultAction(Event& event).
More specifically setting the class and removing it again triggers a cascade of update calls through the DOM tree, calling through:
Element::Update()
Element::UpdateProperties()
ElementStyle::UpdateDefinition()
StyleSheet::GetElementDefiniton()
StyleSheetNode::IsApplicable()
...where it checks the classnames and pseudoclassnames.
For a little more context I am using the lib on a mobile device, so this is happening in response to touch events - repeatedly tapping the screen drives CPU usage up to 50-60%. Not setting the pseudoclass brings CPU usage down to 8%. I think to repro this you would need to click pretty wildly, but worth having a look into!

Afaict this is being caused by a massive number of calls into IsClassSet() (not so much IsPseudoClassSet() but I guess that's because they appear less in my StyleSheetNodes) when it runs through all the StyleSheetNodes multiple times in an update call.