Didn’t have the time to do a very serious benchmark but in case this is useful:

Some SSD, unencrypted
$ dd if=/dev/zero of=zero bs=500M count=1
1+0 records in
1+0 records out
524288000 bytes (524 MB) copied, 4.42172 s, 119 MB/s
Some SSD, with EFS
$ dd if=/dev/zero of=zero bs=500M count=1
1+0 records in
1+0 records out
524288000 bytes (524 MB) copied, 13.4115 s, 39.1 MB/s
Some SATA HDD, unencrypted
$ dd if=/dev/zero of=zero bs=500M count=1
1+0 records in
1+0 records out
524288000 bytes (524 MB) copied, 8.64324 s, 60.7 MB/s
Some SATA HDD, BoxCryptor
$ dd if=/dev/zero of=zero bs=500M count=1
1+0 records in
1+0 records out
524288000 bytes (524 MB) copied, 17.8249 s, 29.4 MB/s
Another run using a Mac with some SSD hard disk. CoreStorage vs. TrueCrypt vs BoxCryptor:
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Unencrypted, SSD
$ dd if=/dev/zero of=zero bs=500m count=1
1+0 records in
1+0 records out
524288000 bytes transferred in 1.431038 secs (366 MB/sec)
CoreStorage (AES-XTS, default), SSD
$ dd if=/dev/zero of=zero bs=500m count=1
1+0 records in
1+0 records out
524288000 bytes transferred in 3.975945 secs (131 MB/sec)
TrueCrypt (AES-XTS, default), SSD
$ dd if=/dev/zero of=zero bs=500m count=1
1+0 records in
1+0 records out
524288000 bytes transferred in 3.984627 secs (131 MB/sec)
BoxCryptor, SSD
$ dd if=/dev/zero of=zero bs=500m count=1
1+0 records in
1+0 records out
524288000 bytes transferred in 8.353867 secs (62 MB/sec)