As another data point, I tested with a regular non-SSD hard drive via USB3.0 and two volumes, one an encrypted partition and the other an encrypted sparseimage on an unencrypted partition.
For large file transfers to the encrypted partition, the speed topped out at about 50MB/s, while transfers to the sparseimage topped out at about 2.5MB/s.
So that's about a 20x performance difference which seems pretty extreme but was fairly consistent in my non-scientific tests. Part of the problem could have been that the sparesimage was having to expand itself during the copy operations, but it still seems pretty slow.
As a reference, copies to the unencrypted partition ran around 70MB/s.
2.5 MB/s encrypted sparseimage on unencrypted partition
50.0 MB/s encyrpted partition
70.0 MB/s unencrypted partition