It is not only about building them. It is also about having a highly capable and trained crew and officers to man each submarine.but there must be bottlenecks elsewhere in the production line? Reactors?
China could produce submarines like sausages but where is the trained crew in huge numbers is going to come from?
There is also the issue of having enough maintenance facilities which must also have a highly-trained workforce.
To conclude, it is important to consider other factors other than "just" quickly building submarines (and other ships wrt PLAN schedule for building ships)