Advisor says I shouldn't expect to finish thesis soon, but won't offer specific feedback on what is missing; how to react?
My PhD advisor once told me that one aspect he considers when rating a thesis is how much the candidate did try to go beyond the initial idea or goal. If they are like "I solved the initial problem, here's the thesis, I just want to finish a fast as possible" then they will earn an intermediate grade at the end. Only if they try to go beyond the initial problem, try to at least estimate the further implications or steps or apply the outcome of the initial problem to further problems, they really can earn a top grade.

To my experience, to successfully finish a physics PhD within roughly a year, you have to have had really great luck to find something really excitingly new that provided a huge progress to your specific field. In most cases after one year you can't even tell for sure what the final focus of the thesis will be. Those applications of the theory to some example cases can easily take up more time than the whole development of the theory as there might come up issues and flaws that you were not expecting. Also their importance is not necessarily reflected in the number of pages they make up in the final thesis.

Therefore it seems to me, that you are somewhat belonging to the first type of people mentioned above who are just heading to leave before really having understood the implications of what they were working on.