I use one wiki for calendar/todo/learning-notes, so it is basically all-in-one, and I’ve join The 5,000 Tiddler Club – People with the Need for Speed,and works fine.
Don’t split wiki due to performance issue, because it can be resolved during development.
But you can choose to split wiki, if you find frequent title / search result conflicts, which can’t be mitigated easily. (though we can filter by tag when searching…So there will be no reason for split wikis…)