The Raspberri Pi is an amazing little machine, but some things are just too heavy on its processor. A couple of these things might stack up when you're copying files to your Pi over the network. This article explains how the slow performance appears, give you instructions which methods to use for best performance and also helps you ruling out a hardware defect.
1. High CPU load preventing faster network throughput
2. The suspects
3. The best performance scenario
4. Boiling it down
5. Further checks if all this didn't help
6. Checking the ethernet connection quality
7. Maybe you need to RMA it (this is very very rare)