https://en.wikipedia.org/wiki/Q15X25Q15X25 is a digital signal processor-intensive mode designed to pass AX.25 packets on HF with speed and reliability much greater than traditional HF ARQ modems. It uses 15 QPSK modulated carriers separated by 125 Hertz, each modulated at 83.333 baud. Q15X25 uses forward error correction (FEC), and like MT63, uses time- and frequency-interleaving in order to avoid most error sources. The raw transmission data rate is typically 2500 bit/s.
So 6.67 minutes to download a 1MB block at 2.5kbps. You'd need to operate on two channels so that you had a 2.5kbps receive and 2.5kbps transmit at the same time.