It is J-20 development which was extremely fast actually. J-35 development has proceeded at a pretty steady clip. I wouldn't call it slow either. Would not be surprised if it was ready when the 3rd carrier enters service. Although the original plan was likely to put the J-15T in it.
Agreed, but even for this type - and I hope not to derail this thread to far off-topic - we lack almost all: AFAIR we saw one yellow aircraft in February 2021 allegedly in final assembly and later on we only got three images at all. So in fact for a type that is expected to enter service before the J-35 and even more so quite soon, I'm surprised we have almost nothing!
So where are all these J-15B's? Are there in fact none and all my / our expectations are wrong or are they hidden so cleverly?