I went into Singapore during a 14 hour layover Monday -> Tuesday.
It was simple, easy, and quick. I had filled out the Singapore Arrival Card thing online before arriving, and I had attached my rapid antigen test (not needed any more I believe, I was there on the last day before that restriction was lifted) and my vaccination certificate, etc. Border control was quick and easy and the queues were well managed. I think it was about half an hour from my flight arriving until I was out landside getting on a train.
Personally after my experience I would personally recommend filling in the Singapore Arrival Card ahead of time. I could attach the PDFs I already had of my vaccination and PCR status. I *think*, but can't remember, that I took a photo of the information page from my passport. Basically the border agent processing me mostly worked from a digital copy of the Arrival Card that he summoned onto his computer. More info linked to from here:
https://www.ica.gov.sg/enter-depart/...a_requirements