I mixed 50 ml of developer from sulfite and concentrate, and timed how long it took for visible cloudiness to appear when back-lit by a bare bulb. Temperature was 30C (summer and no A/C), but I think temperature does not affect the rate of precipitation-reactions, so the times below probably won't change with temperature.
After 15 minutes: Developer was still perfectly clear.
After 20 minutes: Faint cloudiness visible. It looked clear under normal lighting.
After 30 minutes: More cloudiness, but solution still looked clear under normal lighting.
After one hour: Cloudiness is still staying in suspension (not falling to the bottom). Looks slightly cloudy under normal lighting.
My conclusion: With hard water, use the developer immediately after mixing. Or use purified water.
BTW, I examined the test-strip developed with tap-water under the 22x loupe, and saw nothing unusual. I waited around 10 minutes before developing for 9 minutes, so I just missed the beginning of cloudiness. Also, in a different experiment, adding 10 g/L of sodium metaborate to tap-water never produced any cloudiness.
I'll be exploring chelation agents. Thanks again to Rudeofus for suggesting that tap-water be tested. I had overlooked that. Do the chemists here have any comment about these tests and results? Any idea how severe the cloudiness must be before it starts affecting negatives?