Chunk Generation is very slow (like really slow) and using very little CPU doing it

When playing 1.16-pre2, it's really slow generating chunks - going for a trip out from home base involved a lot of stopping at the generated edge and waiting up to 10 seconds for the chunks around me to be generated. 

For what it's worth, I'm playing a world created in 1.16 20w20b, and also played in 20w22a before updating to 1.16-pre2

It at least feels like a generation thing, rather than loading, as on the path back, I didn't have the same issues where I had been before, so loading (I guess) is working OK. I also just re-traced all the way to the stronghold and back (3000 block return trip), and had no chunk loading issues.

It also feels like the server is using a pile less CPU than usual... Typically when it's busy doing generation stuff, it'll pin a single core (of 6) flat out - but I'm not seeing much CPU use at all at the moment.