SF would be extremely behind if it takes years for a single ship to be constructed.
So NASA is "extremely behind" when it takes years for a single shuttle orbiter to be constructed (even when the prototyping stage is long past)? Clearly, a shuttle is smaller than a skyscraper, which may be erected in months. And a bicycle can be constructed in a matter of hours. So yes, obviously NASA is "extremely behind"...
We don't have good points of comparison to estimate the complexity of building a starship. It might be that today's tooling could achieve the feat in five decades or five centuries, so five years is actually pretty good going.
And it's not as if the time to build a capital warship has ever varied much. What has been won by adopting metal hulls that don't require long seasoning has been lost in the lead times required for producing key steel elements such as gunbarrels; what has been won by adopting powered cranes and welding is lost in the increasing bulk of items to be lifted and the increasing precision required of the joinings.
So it would be perfectly justifiable to argue that 24th century dockyards can only do a starship per five years even when they could do five thousand of today's warships in the same time, if we assume that the work is correspondingly more demanding. Or, alternatively or in addition, that it is impossible to build a 24th century starship in any time shorter than five years regardless of construction resources, just as it would be impossible to build a wooden sailing warship any faster today than two centuries ago, because of the basic nature of the construction process.
If starships really could be built in a very short time, an obvious question would then arise: why is Starfleet short of ships, and using old ones? If it only takes half a year to churn out an
Intrepid, Starfleet should by all rights already have a thousand of them, and zero
Mirandas.
If, OTOH, the technology dictates slow and painstaking construction, then it would be worthwhile to adopt a culture of nurturing one's old ships for as long as possible. It's difficult to say what the breaking point would be: should a construction time of two years favor constant newbuilding or sustaining of existing assets? But we know that Starfleet does favor sustaining of existing assets, so the higher our guesstimate for the construction time, the better.
Timo Saloniemi