Slow speeds when moving data to an encrypted image. Method for speeding it up?
Of course writing on an encrypted partition is slower than writing on an unencrypted one, because any writing operation goes through the encryption algorithm.
For the same reason, writing on an encrypted disk image is slower than writing on an unencrypted disk image.

But, what you are comparing here is writing on an encrypted
disk image EW(x) (x is the number of bytes to write) which is then written on an unencrypted filesystem W(x), against writing directly on an unencrypted filesystem W(x). And of course:

EW(x) + W(x) &gt; W(x)


If you would like to measure correctly the overhead introduced by encryption you should compare writing directly on an encrypted filesystem against writing directly on an unencrypted filesystem. Then you would be comparing EW(x) and W(x).

Of course:

EW(x) &gt; W(x)


But you would save a double writing on the same filesystem which can't bring you performance or security.