A 20-inch iMac from mid 2007 was recently checked in for repair due to slow internet connectivity. Most of the time, when a user reports a slow connection, it is due to some variable within their home network.
I informed the owner of this, and he opted to have the iMac checked in just to be sure. During the intake process, I asked how the computer connected to the home network, and learned that the iMac was normally hard wired with ethernet.
The first thing I did was plug the iMac into our network. I opened up Safari, and it did seem abnormally slow. Nothing seemed out of the ordinary as far as the connection settings. I opened up Network Utility, and pinged Google.com. The ping time was extremely slow.
I unplugged the ethernet cable, turned AirPort on, and connected the iMac to our public wireless network. I visited the same webpages, and the response time was immensely faster. Pinged Google.com again—much faster. The system was finally accessing the internet at a normal speed. So, the system seemed to have some sort of issue accessing a network through ethernet. I have seen ethernet ports go bad before, but that usually entails a completely non-functional port.
My last test was to take our USB to Ethernet adapter and try to establish a normal internet connection through that. It worked perfectly. I was browsing sites at a very quick and expected load time. My official diagnosis was a damaged, but still partially functional ethernet port. After a replacement of the logic board (where the ethernet port is located), the issue was solved.