What I have found is that Chromecast is slow compared with Smart TV mirroring.
That is because Chromecast sends the full image, so you get a better result.
However, if you have a recent phone like a Samsung S10 you can max the resolution on the phone screen and the resulting mirror on the TV is pretty good. Almost, but not quite as good as Chromecast.
For me the slowness across the home wifi of the full image is a problem.