Why speed of SSD is increased after enabling FileVault?
SSD performance is known to degrade over time, if no or poor garbage collection is enabled or not working. The later can occur if TRIM is inactive, the disk is very full or it has no time to do garbage collection. So if the SSD wants to write data, it will need to delete old data first, slowing down the process.

When encrypting the SSD, all its contents gets re-written and free blocks are liberated. This will result in faster write speeds. Reformatting a disk is an effective way of restoring write speeds.

Just a theory however...