We are flying from SGN to PEK, layover for 1.5 hours before taking a connecting flight to NYC.

I think there is still no transfer lane. I'm transferring as NYC-PEK-BKK. I have to get in to the custom with "transfer within 24 hrs" that took me 20 mins and get in to the security check for 40 min. Mine is Air China.
You should have the boarding pass for NYC ready from Singapore. And if it's crowded, just ask for a fast lane.

Yes, in general, you need at least one hour to finish those transfer procedures. It's still suggested to hurry up.