AES decryption vs encryption speed Let's consider the CTR mode. For a faster encryption/decryption, is it preferable to use the decryption operation of AES, or its encryption ?
Given the choice, it is preferable to use the block encryption operation of AES, since it often faster than block decryption (never slower AFAIK). For this reason, AES-CTR is defined to use the block encryption operation of AES exclusively; that's both for AES-CTR encryption and AES-CTR decryption, which are the same operation except for IV generation/input.

AES block encryption is faster than AES block decryption because:


MixColumns uses a matrix with smaller coefficients than InvMixColumns, thus is simpler to compute (that's particularly true for purely software implementations; hardware implementations sometime use the same number of cycles, including I believe Intel's AES-NI instructions).
during encryption, subkeys are needed in the order they are produced from the key, but during decryption that order is reversed, therefore on implementations (including hardware) that start decryption with the pristine key as input, some preliminary work is necessary before decryption can start (there's no known shortcut).


Here is for example a crypto library performance report (single-thread, software AES) showing speed differences.