Is disk image encryption slower than partition encryption?

I am considering two options to encrypt some data:

• add a new encrypted partition
• add an encrypted disk image

Which option is faster in terms of disk read/write speed? I suspect that an encrypted disk image would have a worse performance than using an encrypted partition, but I am not sure.

An encrypted partition would likely be faster. With a disk image, you're actually accessing two filesystems, one within the disk image, and the one on which the disk image is stored, as well as the technologies needed to read the disk image itself. It seem unlikely that a disk image could ever be faster than full-disk encryption, but you would have to run benchmarks to be certain.

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.

To summarize:

 2.5 MB/s  encrypted sparseimage on unencrypted partition
50.0 MB/s  encyrpted partition
70.0 MB/s  unencrypted partition