Are there any SD cards optimized for single block read/writes?
Smaller capacity SD cards will often perform better for single-block writes, because their erase page size is smaller. When you write to the card, even if you only write a single byte, the card's internal controller may have to read an entire page (often 4K) into a buffer, modify your one byte, erase the Flash page, then write the whole thing back to the Flash memory. So cards with a smaller erase page size will have less of a penalty for small writes at random addresses.

What microcontroller are you using? SdFatLab works on AVRs and does support multiple block read/write operations. Your files must be in contiguous blocks on the SD card to take advantage of it.

Check out some of the discussion and benchmarks at http://elm-chan.org/docs/mmc/mmc_e.html