I notice that the logging rate increases when I disconnect the phone from the meter – down to about 66ms now, which is a big improvement, but still nowhere near what it should be at 4000/64 – 16ms. Still using Class 4.

Is this the expected behaviour – the logging rate improves when the phone is disconnected?

In any case, it proves that the SD card is not the only bottleneck when the phone is connected.