At 100 megabits, full duplex, the technical maximum is around 12.5MB/s. However, you never quite reach 100% efficiency...so you can usually bank on around 10MB/s. So, 1.5GB of data in a 100 megabit network would take around 2.5 minutes.
Are you getting faster speeds when you are accessing the Internet? Can you change out the network cables to see if you have a bad cable?