[...] how fast would the game cards be?
From my reading of it (and I'll admit a complete lack of expertise in this area), the answer seems to be "pretty much as Nintendo wants (or is willing to pay for)." Nintendo don't use flash memory in their game cards, but rather use ROM (or more specifically a custom variant of Macronix XtraROM). ROM behaves quite differently to NAND flash, because unlike NAND, where all data comes through a controller chip, which has to handle wear levelling, block management, caching, etc, when you read data off a ROM game card you're pulling data directly off the ROM chip itself. This makes ROM speed a pretty simple calculation based on the speed of the chip and the interface to the chip (much like RAM).
Macronix's 45nm XtraROM can currently achieve a read cycle time of 25ns (or 40MHz), and, going by past trends, we could expect their upcoming 32nm XtraROM to hit around 16ns (or 62.5MHz), which is what would almost certainly be used if Nintendo were to go the game card route with NX. On a 16-bit parallel interface, that would give Nintendo 125MB/s sequential reads, and although it's tricky to estimate IOPS values (strictly speaking they're only OPS on ROM), the RAM-like interface and low access latency means you should be able to get close to peak read speeds with random access patterns. For 4K random reads (a typical measure) you'd be looking at possibly 25,000 IOPS.
If they switched to a serial interface, the bandwidth could push up much higher. With 32-bit reads over an 8-bit, 250MHz serial interface they could get 250MB/s sequential reads and potentially 50,000 IOPS. Even 64-bit reads over a 500MHz interface, giving 500MB/s sequential and 100,000 IOPS would probably be technically possible if Nintendo wanted it. Moving to higher speed interfaces would certainly add to the cost, but only by a fairly small amount, and the matter will largely be a question of what Nintendo feels is enough. Given that a 16-bit parallel interface places the game cards firmly above PS4 and XBO's internal HDDs in terms of performance, that may well be enough for Nintendo.
