To speed up the conversion process, you could try making the temporary file a WAV file rather than ALAC; it will require more disk space, but it should be quicker to process (no decoding required). To write a WAV file, you can use BASS_Encode_Start, like this...
BASS_Encode_Start(handle, wavfilename, BASS_ENCODE_PCM|BASS_ENCODE_AUTOFREE, 0, 0);