Very Slow Encrypt?

I am using the SFL libraries to encrypt some data. Currently I have been
using the RC2-CBC encryption algorithm and I have found it to be very very
slow (approx. 360seconds for 400K of data). So I stepped through the code
and the reason was this;

The RC2-CBC algorithm uses a block size of 8 bytes and whenever the buffer
it is holding the encrypted data in is full it will expand it by 8 bytes.
This means calloc-ing some memory and copying it into the new buffer. Now
for a large file (say 2Meg) this means it has to do about 250,000 calloc's
and memcpy's. By changing this and forcing it to increase the buffer by
1024bytes each time I found a speed increase of about 3000% ( 1 Meg would
now be encrypted in 31seconds as opposed to the 24+ minutes it took
before ).

The code which I noticed this in focused on the

void CSM_Free3::RawEncrypt(CSM_Buffer *pbufInput, CSM_Buffer *pbufOutput,
         Filter *pCBCEncryption, int iINBlockLen)

function, which is in sm_free3.cpp. This adds the encrypted data to the
CSM_Buffer using the Write function;

SME(pbufOutput->Write(&achOut[0], getLength));

The getLength variable passed in is the length of the new data to add
(always 8 for RC2-CBC). Now if you look at the Write function,
sm_buffer.cpp, you will see that the amount of extra memory allocated is
based on the length passed in, hence only 8 bytes.

Am I missing something here?
Is there a reason why it is done this way?
Has anyone else noticed this speed issue and found a different way to speed
the process up?

Regards

William Adams
Software Engineer
Nexor.
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Tel:  +44 115 9535536
Fax: +44 115 9520519