Breakfast is the battleground du jour in the restaurant industry, with everyone from Taco Bell to White Castle piling on. For McDonald's Corp., the pioneer of fast-food breakfast, it is one more area where rivals are gaining traction on its turf.

The burger giant has been struggling to maintain relevance among younger consumers, to fill orders quickly in kitchens that have grown overwhelmed with menu items, and to keep up its standards for service with a smile. While McDonald's is busy trying to correct those issues, competitors...