Added a wiki with what info I could find on rail links from the airport.

The initial rail links look a bit underwhelming for pax heading to the city center. Perhaps a bit faster than T3 to city center, but not hugely so. Looks like Daxing to Tian'anmen (Jing-xiong'an HSR -> Line 9 -> Line 1) will take 40 minutes (plus wait/transfer times). Taking the subway from Daxing via Caoqiao to Tian'anmen should take around 55 minutes.

Currently, PEK T3 to Tian'anmen takes about 45 minutes (and 30 min in reverse).

I'm curious to know how long the direct rail links between PEK and PKX will take when they're complete.