Thank you again everybody for your help and advice! Reporting on my experience this morning. According to the Schiphol website, it's supposed to be a "peak day," which I'm guessing most Sundays are. I am non-priority. My flight (KLM to Washington DC: so Departure Terminal 2, check-in at 8-16, gate E6) is at 1:05pm, and I took the 8:35am train from Amsterdam Centraal, which got to Schiphol at about 8:52am. The train was very empty. No line to get into the airport. There was a sign saying not to enter more than 4 hours in advance in front of the inclined travelators, but the two guys there checked no one's boarding pass at all as far as I could tell, possibly because it wasn't busy. I didn't have any bags to check but the check-in counter line looked "normal" in length -- it didn't fill the entire winding part in front of the check-in counter. I got into the security line at 9:03 and was done with it at about 9:55. There were 6 scanners open, I think, and a separate line at the end (I'm guessing for priority passengers) that probably had a wait of 10 minutes or less. The other line I could see (is that for Departures 1?) looked a bit worse - people were being held on the entrance floor and not climbing the stairs after scanning their boarding pass to be let into the line - but it was mildly worse as opposed to dramatically different.
As I'm guessing is typical, the passport control for US citizens was self-scanning and took maybe two minutes. Only one person was in front of me.
So the whole thing from trying to enter the airport to getting out after security/passport was about an hour. I had more trouble leaving Dulles (90 minute line to check into the KLM counter to have my passport looked at and 50 minutes for security, so 2 hours and 20 minutes total). Although I now have plenty of time to kill, I'm just happy that it was easy and straightforward and zero stress. And I have time to write this message! I was probably helped by the fact that this seems like a quieter time here. There were maybe 15-20 people in my train car. And it seems like there are plenty of places to sit in the terminal. The early morning might have been much worse.
It also appeared from looking at the departures board that only a handful of flights were canceled (including the earlier morning flights). The weather is great, which might be helping.
I also wanted to add that passport control when I arrived (around 7:30am on Tues, Jul 5) was pretty "normal," also, although there was a sign saying that the lines might be long. Other American and Canadian citizens at my meeting, who were on different flights but I think they all took overnight flights, also said that they didn't have an unusually long line at passport control on arrival. So that part of things may have gotten a bit better, at least far as early morning arrivals go.