If 80% of the code is yours, of course you will be able to work with it faster than everyone else. It's easier to write code than read others' code; you already know how it all works, where the bodies are buried, etc.; and you probably know the other 20% pretty well too because the parts you aren't writing yourself you're reviewing and understanding in the "big picture" context better than anyone else. Not to mention getting pulled into meetings and hallway discussions that give you insights into what's coming next. Long story short: You've amassed company & product specific intellectual capital.
You're being challenged, it's just not a technical challenge. You need to define for yourself what you want out of this situation. You can of course go find another job as others have suggested; I'm sure that will feel new and technically challenging for a while at least!
Or you can accept that your challenge now is to figure out why you feel stressed and alone; why your team seems so slow; why your response is to be more controlling; why you don't trust them to get the job done; and why they just obey your edicts rather than arguing and sharing their own ideas. These are not technical problems, but they are common when engineers take on leadership roles. Is there camaraderie on the team? Can you gently suss out their thoughts on the matter? What are their goals? Try letting them solve the problem; resist urges to take control. Set limits on yourself so instead of taking over with coding, you start to learn why they're slow, and help them improve (are they over engineering? getting stuck and not asking for advice?). Allow them to make mistakes, and develop a culture of sharing in success and failure and learn from both. Recognize that they won't be as efficient given random tasks as you, since you know almost all of it; help them build up their own areas of expertise, and create a "map" of experts so they can help each other directly instead of working through you.
Most importantly, stop measuring your commit count, task count, etc., unless that's really what you want to maximize... in which case, yes, the rest of your team is going to be slower than you, depend on your understanding, and probably not have much hope of surpassing your performance. Instead, focus more on the team's performance, the project's success, etc.; find ways you can measure those if you must; and discuss with your boss so he sees your efforts, can give feedback & advice, and so you both know how you're valued.