It takes two days at around 100 to 120 km/h over 675 bridges, though some recommend [2] that you spend a day or so in Xining on the way to acclimatise to the altitude.
Every train carries a doctor and oxygen to increase the concentration of oxygen in the carriages from 21% to 25% (but no pressurisation), plus a separate oxygen supply for each passenger.
I'm not a fan of China, but its rail network is something to behold.
On the other hand that distance is a lot smaller than Chicago-Seattle (4x) so the lack of high speed in the alpine region isn't as noticable for a night train because the distances are much shorter. The thing about sleeper trains is that you need the distance to be 8-15 hours. If it's less than 8h you'd get uncomfortable departure or arrival times. 10h for Zurich-Vienna is perfect for a sleeper.
Unfortunately Germany is the only neighbor with an extensive high-speed rail network that Austria has.
It would be cool if there were a way for me to leave Seattle on Sunday night and arrive in Chicago on Monday morning having gotten a full night of sleep.
The country is roughly 300km long and ~200km high, add to that a lot of cities, which need to be serviced with relatively small distances.
HSR only makes sense, when a train can travel big distances between nodes.
Instead a different approach was chosen:
The trains are dispatched by clock face scheduling[1], which makes much more sense. Trains leave nodes by the (usually) half hour and deliver passengers to connecting nodes within the time required to connect to transfer. This guarantees scheduled connections, even when there's no direct train.
For example: If you want to get from Zurich to Lugano there are trains leaving at 5:10pm, 6:10pm, 7:10pm, etc.
The first and the last are direct trains, with the 6:10 you need to get out at Arth Goldau to connect with another train, which leaves 4 minutes later.
For a small country like Switzerland this concept makes far more sense than HSR.
It's probably one of the most efficient train networks with the exception of Japan. So bemoaning generally happens on a very high level, if at all.
Not really doable for a lot of Austria and generally the Alpine regions.
There are not many high speed tracks in Austria and Switzerland, the reason being that the countries are relatively small and that building high speed tracks through the alps is insanely expensive.
Switzerland built two base tunnels through the alps: The Lötschberg[1] - and Gotthard[2] base tunnels, which are geared to a top speed of 250 km/h (ca. 155 miles/h) and a normal travel speed of 200 km/h (125).
That's why it takes roughly 10 hours to get from Zurich to Vienna, but only 4 to get from Zurich to Paris (after Mulhouse the whole track is high speed capable and trains run with up to 320 kmh (200 m) on that track.
Edited to add: Of course night trains make a lot of sense on that route.
[1] https://en.wikipedia.org/wiki/L%C3%B6tschberg_Base_Tunnel
[2] https://en.wikipedia.org/wiki/Gotthard_Base_Tunnel