Millions of polygons: displaying layers in Leaflet based on location being viewed?

I'm trying to create a map of all parcels in a state. In my state, this is several million parcels.

In their current format, they are encoded as topojson and divided up by county. I'd like to show only a certain county's parcels at a time, preferably when the user is zoomed in on that county.

Is there a way to achieve this? I'm open to alternative map libraries / ideas.