I've noticed in using my external SDXC cards that the write speed tends to decrease over time during a large file transfer.
For example, when an SDXC card is plugged into the port on my 15" rMBP and I transfer a 90gb file from the internal SSD, it starts around 50 mb/sec and drops gradually to around 100 kb/sec.
The cards in question are SanDisk Ultra 128gb micoSDXC card, and is read using the SD card adapter. It benchmarks at around 50 mb/sec write and 80 mb/sec read. The card is formatted as exFAT with Apple Partition Map.
What causes the write speed slowdown? Is it possible to avoid?