encrypting large file taking too much time

Question

I am trying to encrypt and decrypt a file using aes 128. Everything works fine but it takes too much time to encrypt a file which is just 165 kb. I checked the whole internet and tried to use different tactics but the result is still same takes 1 minute to encrypt a 165kb file.

here is how I do the reading and encrypting

string keyStr = "abcdefghijklmnop";
byte key[16];
charToByte(key, keyStr.c_str());
//Key expansion  
word w[4 * (Nr + 1)];
KeyExpansion(key, w);

bitset<128> data;
byte plain[16];

//Encrypt the file s.csv to s.csv.enc  
ifstream in;
ofstream out;
in.open("s.csv", ios::binary);
out.open("s.csv.enc", ios::binary);

while (in.read((char*)&data, sizeof(data)))
{
    divideToByte(plain, data);
    encrypt(plain, w);
    data = mergeByte(plain);
    out.write((char*)&data, sizeof(data));
    data.reset();  //Set 0  
}
in.close();
out.close();

I took the code from: https://programmer.group/c-implementation-of-aes-encryption-algorithms.html

any help is appreciated.