AES of openABE's performance is not normal

[sploving]

When encrypt and decrypt large files using AES of openABE, it costs a lot of time than normal AES. Here is the performance I debugeed:
decrypt: EVP_DecryptInit_ex0.002 ms
decrypt: EVP_DecryptUpdate 1 0 ms
decrypt: EVP_DecryptUpdate 2 50.797 ms
decrypt: pt_buf.appendArray 89.899 ms
decrypt: EVP_CIPHER_CTX_ctrl0.002 ms
decrypt: pt_buf.toString() 2006.45 ms
decrypt: 2212.04 ms

This is because openABE use its own string type "OpenABEByteString". Its toString API and appendArray cost a lot of time.

So I have a question: in the following API

OpenABE_ERROR
OpenABESymKeyAuthEnc::encrypt(const string& plaintext, OpenABEByteString *iv, OpenABEByteString *ciphertext, OpenABEByteString *tag)
{

bool
OpenABESymKeyAuthEnc::decrypt(string& plaintext, OpenABEByteString* iv, OpenABEByteString* ciphertext, OpenABEByteString* tag)
{

Why not use standard string instead of OpenABEByteString?

Thanks in advance