I am copying files from one powerful computer to another on my home network. The files are copying slower than what it would take to download them over the internet. If my connection is supposed to be like 300mbps on wireless-n compared to 22mbs over the cable internet connection, why can't I copy files faster than it?
First of all, most desktop computers aren't capable of copying files faster than 15-20 MB/s (mega bytes, big B) or 120-160 Mb/s (mega bits, little b). The upper limit is due to a combination of hard disk speed, I/O bus bandwidth, available memory, and most importantly, processor usage and disk activity from other programs. In particular, if you have a virus scanner that has a "scan on access" feature (on either end), that will kill throughput.
Secondly, and most obviously, your router configuration and wireless signal quality can have a significant impact on throughput.
Thirdly, what you are transferring makes a difference too. One big file will generally transfer faster than a zillion small files (of equivalent size).
Your router might be operating in mixed mode, and not actually operating in 802.11n mode.
There might be interference from another nearby wireless router. Try adjusting the channel.
You might not be close enough to the wireless router to get a good signal.
Other traffic might be competing on your LAN or WLAN. If you are running Bittorrent, for example, it can slow down local transfers.
Remember that 300Mbps (megabits per second) is equivalent to 37.5MBps (megabytes per second). Make sure you are not confusing these measurements.