Chicken stock in slow cooker - danger zone?

It sounds like something is wrong with your slow cooker. On low, it should hit somewhere in the 190s, on high in the low 200s. On high, I'd expect it to get there fairly quickly—within two hours. Both should get you to 140 within an hour.

That said, the actual safety temperature (with a good thermometer) is 130 or 131F. 135 or 140F leave some safety margin for not completely accurate thermometers, maybe missing the coldest part of the food by a little bit, etc. And the standard is you have 2 hours to get there (used to be 4).

So, unfortunately, "it seemed like 2–3 hours" makes it really hard to answer if you exceeded that. I'm not qualified to tell you exactly how much risk (if any) you'd be taking if it took 3h.

But I can make some suggestions for keeping the time to 140F down:

1. Start with warmer water. There is a trade-off; hot water is supposed to make a more cloudy stock. If you simmered the water on the stove first (or in a tea kettle, etc.), then your time to 140 will be very short. Even starting with room temperature water instead of cold water is ten or twenty degrees closer. You could also, as Joe suggests, start with less water and add hot water later on to bring it up to the desired quantity (but don't underfill the slow cooker).

2. Make sure your ingredients are defrosted (in the fridge). Melting something takes extra energy; this is known as the enthalpy or heat of fusion. For ice, it's a lot of energy—going from ?32°F ice to ?32°F water is about as much energy as it'll take to get it from there to 175°F.

3. If nothing else works, bring it to a simmer in a stock pot then transfer it over. Unfortunately you'll have to wash a second pot.

Or, finally, you can make very good chicken stock in a pressure cooker, in under an hour total cook time. But of course you need to keep an eye on that.