关于VC中AES算法加解密速度问题

我是一个刚学VC的菜鸟，找了一个AES加解密文件的程序，加解密小文件的时候速度不错，可是几M或几十M的文件的时候速度很慢，该怎么修改程序啊
加密程序如下：
if(m_filename=="")return;
FILE* ff;
FILE* fp;
ff=fopen((LPCTSTR)m_filename,"rb");
if(!ff)return;
fseek(ff,0,SEEK_END);
long lFileLen=ftell(ff);
fseek(ff,0,SEEK_SET);
long c=lFileLen/16;
long d=lFileLen%16;
m_filename+=".aen";
fp=fopen((LPCTSTR)m_filename,"ab");
if(!fp){fclose(ff);return;}
unsigned char inBuff[25],ouBuff[25];
KAES kaes(24,(unsigned char*)"\x0\x1\x2\x3\x4\x5\x6\x7\x8\x9\xa\xb\xc\xd\xe\xf\x10\x11\x12\x13\x14\x15\x16\x17");
for(long i=0;i<c;i++)
{
fread(inBuff,1,16,ff);
kaes.Cipher(inBuff,ouBuff);
fwrite(ouBuff,1,16,fp);
}
if(d)
{
memset(inBuff,0,16);
c=fread(inBuff,1,d,ff);
kaes.Cipher(inBuff,ouBuff);
fwrite(ouBuff,1,16,fp);
}
fclose(ff);
fclose(fp);
MessageBox("加密成功！");