Hey! That's amazing that you found this.
I haven't benchmarked much, but I think I measured a 15% hit on speedtest1 [1] (with about 10% inside your library). Less if I kept temporary files in memory. Other solutions claim less of a performance hit. I'd have to measure to make sure.
There are some things I could do to improve performance. A partial block update needs a read-update-write cycle; journals and WALs do some of these. I could buffer consecutive writes in memory unencrypted and only flush them on fsync. Didn't do that as it requires some additional bookkeeping.
I don't think Encrypt and Decrypt allocate at all! They encrypt/decrypt in-place; the returned slice just aliases the input block. But thanks, it'd be pretty bad if they did.