SSD and Mountain Lion
Try erasing free space (from Disk Utility, on the bottom left corner of the "erase" tab of your drive), and then enable trim (via Trim Enabler).

More than likely this drop in performance is caused by the SSD having to clear memory before being able to write to it. Flash memory can only bring bits up (from 0 to 1), meaning that over-writing a non-zero byte requires first erasing the whole byte, then bringing up whatever bits are needed. This means that 2 operations are needed (erase, write) rather than just 1 (write). TRIM helps improve performance by actively zeroing out memory locations that have been deleted during idle times, allowing for a single write operation to be sufficient for writing to that location. Erasing free space will do the same thing, but it's a one time deal, whereas TRIM works in the background to keep it all tidy.