Many of the fastest most reliable SD cards for embedded systems are small, 2GB or less, and based on SLC flash. Larger SLC cards are available but are very expensive.
Due to the lower complexity in managing SLC flash, the write performance for SLC at the IC level is typically twice as fast. With controller multiplexing this can be reduced, but this accounts for the main delta in read speed versus write speed in retail flash products today.
SLC flash SDs last 20-100 times longer than consumer grade MLC flash SDs.
3-Bit MLC technology (a.k.a. TLC) has an exponentially lower number of program/erase cycles. SLC technology, targeting embedded and mission critical applications, will continue to maintain an emphasis on endurance and reliability.