Re: "A little eBay shopping and you can find 128GB Micro SD cards for under a tenner"
"Is an SD card as quick as built-in memory? I assume it is, if you get a decent card?"
Not even remotely close, but a decent speed class card is usually fast enough that you won't notice the difference on a phone, unless perhaps if shooting insanely high-square-pixel video directly to card.
AFAIK as a rule of thumb the speed for such devices is internal flash>CF card~USB drive*>>SD card
The internal memory of the iPhone is compact flash, and with a high-speed interface I would expect, so it's expected to be massively faster than a SD card. By a factor 10 at the very least I would guess. Again, for most uses it probably won't be noticeable very often especially on a phone. For example my Pi can stream 720p video seamlessly from a relatively low-speed (class4) SD card.
*both USB devices and CF cards speeds vary enormously depending on the interface, the storage hardware and the iteration, so they leapfrog each other on a regular basis; they are not usually direct competitors though, so the comparison is not really relevant)
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