Can the ship perform more calculations per second since it is much larger and is not confined to the size of an average human? Or is the ship actually a distributed network of many computers specialized for specific tasks, thus making the two difficult to objectively compare?
Interestingly, there is an episode where the ship and Data directly interact with one another on a digital scale - A Fistful of Datas.
In this episode, Georgi and Data attempt to connect Data to the Enterprise's main computer, so that in the event of an emergency Data could take full control of the Enterprise.
Their experiment goes remarkably well, until Data tries to access the ship's secondary systems. Data detects an overload in his positronic matrix and shuts the experiment down, but the damage is done in the form of a Texas-Talkin' Data, as well as Data being represented in several parts of the ship (most notably, as several characters on the Holodeck).
Up until that point though, Data was able to access individual parts of the system and operate them. The sudden surge, however, suggests that some systems may be too complex for him to handle. And the fact that he was accessing systems one at a time also lends credit to suggest the Enterprise's computer is larger and more complex than Data's brain can handle all at once.
This may also indicate a limit on Data's ability to multi-task as well as the ship computer, or that there is simply an incompatibility between the two. Unfortunately, there is no way to tell, because the series does not do a particularly deep analysis into the ship's data processing power.
Regarding Voyager's secondary processor, 575 trillion calculations per nanosecond is actually 575,000 exaflops - with each of these 575,000 exaflops about equivalent to a human brain. In contrast, with 60 petaflops (0.06 exaflops), Data's positronic brain would only be about the equivalent of one mouse brain.
Note from wiki:Exascale computing refers to computing systems capable of at least one exaFLOPS, or a billion billion (i.e. a quintillion) calculations per second. Such capacity represents a thousandfold increase over the first petascale computer that came into operation in 2008. (One exaflop is a thousand petaflops or a quintillion, 1018, double precision floating point operations per second)..., and Exascale computing would be a significant achievement in computer engineering, as an exascale computer would have processing power on the order of the estimated processing power of the human brain at the neural level (although the functional power required to simulate a human brain might be lower).
It was mentioned on Voyager by its own computer that it has transluminal processing. This implies from the wording alone that the main computer processor is FTL based...
And since most (or practically all) technology on a Federation ship is subspace based, even computers would effectively be using subspace principles.
It never made any sense that the Federation was unable to create its own Androids. Heck, given the rate of exponential progression, androids with more processing power than Data will be possible shortly in reality... Star Trek missed the mark on 'exponential development' aspect especially when it had various aliens for example being better at translation than it... that's just impossible. No organic entity would be able to process things faster than a computer... a computer simply needs a proper set of algorithms to work with... and while those used in Trek seemed advanced, they also seemed outdated in comparison to the hardware's capabilities.