You are right that the interface is a 100mbps (Theoritical), but 1.7mbps is very low.
Some parameters worth checking:
1. The duplex settings on the SSG interface connecting to the ISP MODEM and the MODEM duplex setting. If it is on autosense, force it to full-100M
2. Check the itnerface counters of the firewall for any error counter increment
3. Ensure the MTU is matched between the MODEM and the SSG