A 3-to 4-pound pot roast takes approximately 4.5 to 5.5 hours to cook in a slow cooker on a high setting, and it takes the roast 9 to 11 hours to cook on the low setting. The Department of Agriculture suggests that food be cooked for at least an hour on high before switching to the cooker's low setting to ensure it is cooked thoroughly.
It is important to heat meat to 140 degrees Fahrenheit as quickly as possible in order to kill any bacteria that may be present. A pot roast is done when it reaches an internal temperature of at least 145 degrees Fahrenheit. Cooking pot roast at a higher temperature requires a shorter cooking time, but the meat might have a chewy texture. Higher temperatures cause meat fibers to contract and become tough as they boil, which forces juices out of the meat. Cooking pot roast at a lower temperature ensures that the meat is tender. A slow simmer allows meat fibers to expand and retain moisture while soaking up additional moisture from the stock in the slow cooker. This is especially true for less expensive cuts of meat, such as brisket, rump and chuck, that contain the greatest amount of flavor.