I just tested 2 more SD cards.
Here's write speeds from a generic SanDisk 8GB (not "Ultra" - might be a counterfeit):
SD write, us=1435
SD write, us=4199
SD write, us=1516
SD write, us=1423
SD write, us=1451
SD write, us=1427
SD write, us=1461
SD write, us=1433
It seems to perform very well, with only occasional 4.2 ms latency.
Here's measurements from a Transcend 128 MB card:
SD write, us=5284
SD write, us=7102
SD write, us=10068
SD write, us=4811
SD write, us=5911
SD write, us=8421
SD write, us=8177
SD write, us=6035
This card is really terrible. In fact, the average time to write 512 byes is close to 5.8 ms, so this card is marginal for recording 44.1 kHz, 16 bit audio.
But at only 8000 Hz with 8 bits, the latency is still well under 25 ms.