If I cook taco meat at a low heat for a longer time will it be more tender?

You are cooking ground beef. The fat content will be a factor but cooking it slower will not make it any more tender. Cooking it longer may dry it out, so just make sure it is cooked through. If you grind your own sirloin or other cuts, then it might be more tender because the chunks may be larger depending on which wheel you use on your grinder.