Could be old lines, too. Maybe a short here or there? Or it could just be your connection through your ISP.
I know I had a 56K modem for many years, and I was LUCKY if I got upwards of 22.1K.
Most of the time it's the service, the lines, or some other factor, but not the modem itself. The modems are so old there's very little that could be wrong with them, without being blaringly obvious (smoke, flames, whatever)