That would be really easy (though it probably wouldn't be perceptible by humans, but you'd certainly see it if you look at actual CPU and memory usage): just look at some simple webpage that's only static HTML. uBO uses resources, so of course it's going to perform worse not having it there at all. And going through tens of thousands of filter rules isn't exactly a trivial task.

However, at some point, the resources saved (by blocking ads running JS) will outweigh the resources used by the ad-blocker. In typical modern web pages, that bar is probably pretty low, because there's SO much BS advertising and tracking.