If you keep a timer and don't overcook the meat, you can cook beef on high in a slow cooker. However, if you leave it for too long, it may become tough and dry. Likewise, it will also be tough if you don’t cook it long enough. You can test your progress by pricking the beef with a fork every hour or two and gauge tenderness. If the meat is still tough once the cooking time is up, add another hour to the cooking time to make the meat more tender.