Pressure cooking (including the time to bring up to pressure, pressure cooking, and pressure release) can save you about 2-6 of those hours. Depending on who you talk to, it's either almost as good or perhaps better than simmering for several hours.
If you're willing to wait the extra half hour for a slow pressure release, then you get a beautiful super-clear consommé-style stock because the pressurized environment means the liquid never boils. I've never achieved that in a stock pot, even if i'm being finicky about temperature control.