I was copying large video files from an older Dell Optiplex 745 yesterday (to a QNAP TS453b NAS), and had diabolical throughput, i.e. <10% of Gb connection when viewed in task manager. On another PC, I could max out Gb connection to the same NAS. Hmm.

So, I checked the driver, installed the latest from the Dell website (dated 2014), still [censored]. Then in Device Manager I selected "search for updated driver from web", and low and behold the throughput jumped up to near maximum as shown in task manager. The installed driver was still ~6 years old, but it made a massive difference. CPU utilisation was very low.

Network interface is Broadcom 57xx Gigabit integrated controller, OS is Win7 32-bit on that PC.