According to my tests, I take a 18.89% performance hit because of using ecryptfs. I understand this is not much, but just wanted to test and document it anyway.
Hard Disk Model: Western Digital, WDC WD5000LPVX-2, 500GB (465GiB)
##With ecryptfs
$ time dd if=/dev/zero of=filename bs=1024 count=1000000
1000000+0 records in
1000000+0 records out
1024000000 bytes (1.0 GB) copied, 13.7076 s, 74.7 MB/s
real 13.735 user 0.088 sys 9.298 pcpu 68.33
##Without ecryptfs (in /tmp)
$ time dd if=/dev/zero of=filename bs=1024 count=1000000
1000000+0 records in
1000000+0 records out
1024000000 bytes (1.0 GB) copied, 11.1235 s, 92.1 MB/s
real 11.136 user 0.099 sys 1.423 pcpu 13.67