Further below is how the 251GB MacBook Air solid state drive (SSD) performs over 95% of its capacity (an 11GB system was installed).
Apple is not truthful about the SSD capacity; it formats to less than 250.66GB, yet is advertised as a 256GB drive.
Most likely the missing 5+ gigabytes are used for some internal housekeeping. But if that were the case, then OWC could call their 400GB drive a 512GB, their 200GB a 256GB, etc.
I consider this false and misleading advertising, even if the difference is only 2%.
The first thing I did was to wipe out the Apple system install, and install a minimal one, which totalled about 9.22GB.
Performance on the Apple SSD is outstanding compared to hard drives, which at their best would top out at about 100MB/sec and then steadily decline (read Why You Need More Space Than You Need). But the performance is below that of top-grade SSDs, like the OWC Mercury Extreme Pro.
Note the regular downward spikes in performance due to some kind of internal housekeeping. This is with a brand-new drive; of some concern is how well it holds up over time. See my Severe Duty Test of the Apple 512GB SSD.
The 251GB SSD in the MacBook Air appears to perform a bit less well than the 512GB (500GB) SSD offered for the Mac Pro. But both drives show the same pattern of regular downward spikes, including a nasty glitch at about the half-way mark. How well this SSD will hold up over time remains to be seen. I won’t be toasting mine in any severe duty test.