Extremely slow write speed to encrypted external drive on Mavericks
I have the same problem, which I'm pretty sure is due to a combination of the way write operations work on flash memory and the way core storage (or any whole-volume) encryption works.

First, write behavior: unlike volatile memory (the stuff used in the memory of your computer) or hard disks, where any bit can be written to 0 or 1 at any time, flash memory has two main states: written and erased. Within "written" are 0 and 1. When you need to write to flash memory you must write an entire block that is currently in the erased state. File system software in the OS may know what blocks are free, but the controller and storage on a flash device don't. A special way for the OS to tell an SSD to make blocks available has been devised for "bus-connected" SSDs: it's called TRIM. USB protocol stacks don't, to my knowledge, support TRIM. So, basically, flash memory continues to fill up until there are no actual erased blocks, at which point the file system has to erase and rewrite blocks by reading them, merging in the new data, erasing, and writing them back out. That's why you see small-file write performance degrade on SSDs over time.

The special circumstances of encrypted volumes is interesting: depending on the way the encryption works, it may actually encrypt an entire volume, filling all the blocks with what appears to be random data even if the blocks are actually unused and would contain zeroes. So when you turn on FileVault (or otherwise enable core storage encryption), it basically consumes the entire volume, leaving no space for write operations. The file system has to constantly read, erase and rewrite blocks so that it can rewrite them with any encrypted data you want to put on it.

Now I will say right here that this is speculation based on a reasonable understanding of how things work, but there are people who actually know the details, who may correct or improve my explanation and I hope that they will do so.