I didn't think it took longer than usual scans, but my scans were always fairly detailed. I had 2 NT scans; one with my regular OB and then 2 days later I had one with my perinatologist. My OB always told me exactly what he was looking at and showed me various things while scanning. At the perinatologist, the tech scanning would occasionally point out things, but it was more typical of her to scan in mostly silence and then the perinatologist would come in and scan himself and tell me what he was seeing as he scanned. I will say though that most techs will not say much during scans. The doctor is the one who holds the liability insurance and so the techs leave it up to the doctors to explain anything.
And, of course, if something unexpected comes up, it would be stressful and worrying. I was high risk for something to be wrong with baby anyway, which is why I was seeing a perinatologist. During my 12 week scan, there was a bubble looking thing near the umbilical cord. The tech first noticed it and when I asked what it was she said she didn't know. When the perinatologist scanned me I asked him and, much to my horror, he said he didn't know and hadn't see it before. By the time the scan ended, he had decided it was probably a cyst on the umbilical cord, an indication of chromosome problems. I was already placed at a 1:11 risk for trisomy 13 and 18 and 1:17 for Down syndrome and this was before the scan. So yes, it was extremely stressful. All turned out fine with baby though.
I'm glad I had the scan and then went to have the MaterniT21 test, which tests for things such as trisomy 13, 18 and 21. I am one of those people who believe knowledge is power and I feel more in control the more info I have. I wanted to be able to prepare myself if something had been wrong, and would have wanted my doctors aware as well in case something needed to be done right after birth. Plus, I wanted to deliver in a small community hospital, and if baby had something wrong I'd have needed to deliver in a larger, better equipped hospital.
Sorry for the book. Oh, and I see some people refer to the test as having a high false positive rate, but that's not possible because this test doesn't ever tell you that something is wrong with baby, it only gives a ratio. Even if you were to get, say a 1:2 risk that somethig is wrong, doens't mean something is. You have a 50% chance that nothing is. Good luck and hope you have a wonderful scan with absolutely nothing to worry about!