Observable did not handle my rendering requirements involving several hundreds of data points for my parallel coordinate plot. That said I'm not sure how it would fare outside of the notebook setting.
It should be as fast as equivalent visualization done in the frontend of a web page; after all, what you do in Observable gets directly executed by your browser. I wonder what your particular requirements are. On my end, I had acceptable performance on interactive prototypes involving updating multiple histograms, where inputs had tens of thousands of points.