Writing to a fully encrypted USB flash drive on Mavericks is extremely slow.
The drive model I used for testing was a Kingston DataTraveler Ultimate 3.0 G3 (64 GB). I tested transfer speeds by reading/writing a large file from/to both an encrypted and an unencrypted drive. I tested on a current Macbook with Mavericks and on an older machine with Mountain Lion. I used Disk Utility to format the drive as Mac OS Extended (Journaled) and Mac OS Extended (Journaled, Encrypted).
MacBook Pro (2013) with USB 3.0 running OS X 10.9.2 (13C64) Write: 86.16 MB/sec (encrypted: 0.62 MB/sec)
Read: 181.66 MB/sec (encrypted: 151.15 MB/sec) MacBook Pro (2007) with USB 2.0 running OS X 10.8.5 (12F45) Write: 23.57 MB/sec (encrypted: 5.04 MB/sec)
Read: 36.23 MB/sec (encrypted: 37.87 MB/sec) As you can see on the older machine the write speed clearly decreases when writing to the encrypted volume but is still about ten times faster compared with the newer machine running Mavericks. Could this be some recently introduced issue in FileVault or CoreStorage?
The USB drive seems to have had a hardware defect from the start. I got a replacement drive (same model) which still doesn't deliver the results I would expect but at least the encrypted write speed of the 2013 MBP is now on par with the 2007 MBP.
MacBook Pro (2013) with USB 3.0 running OS X 10.9.3 (13D65)
Write: 135.41 MB/sec (encrypted: 9.29 MB/sec)
Read: 196.22 MB/sec (encrypted: 187.04 MB/sec)
MacBook Pro (2007) with USB 2.0 running OS X 10.8.5 (12F45)
Write: - MB/sec (encrypted: 9.39 MB/sec)
Read: - MB/sec (encrypted: 37.79 MB/sec)
This still leaves the question though why the encrypted write speed to the USB drive on the 2013 MBP is less then ten percent of the regular write speed. I also compared read/write speeds before and after activating FileVault on the internal SSD of the 2013 MBP and there I couldn't detect any slowdown at all.