Okay, well done, the extra info is helpful.
From what you tell us, the problem is at your iMac, so you need to check your System Prefs > Network prefs, proxy settings in particular. Then check System Prefs > Sharing > Firewall. How you set things is up to you, but for simplicity and stability - and, hopefully, your connection speed - have as few facilities checked as possible. I have Personal File Sharing ONLY checked.
50 ft cable - hmm. You can expect a slight drop in speed over that length, but not glaringly so. I think 15m is the recommended maximum, so 50' is neither here nor there.
Next - run a few ping and traceroute tests with Terminal. Do you know how to do that? If not, I'll PM you with the info. These tests are to determine if there's a problem with a server with your ISP, maybe a latency (congestion) issue. I don't think so, because your dad's computer and your laptop don't have this speed problem.
Last thought - have you and your dad considered going wireless?