Why is my Internet connection acting so slow?

[D]

It sounds to me that you are describing the speed of your WiFi connection, not your internet connection. With a connection speed of up to 300 + Mbps I assume you are on an N wireless network. That is the speed that your computer communicates with the wireless router. Your internet connection is at a different speed, generally 1.5 Mbps - 50 Mbps depending on your provider.

A great place to check your actual internet speed is:
http://www.speakeasy.net/speedtest
Use that site and select the nearest city to see what your internet connection speed is running at.

Now, for why your wireless network may be slowing down... that could be for many reasons. Since it is a wireless network it is prone to interference from other devices whether that is cordless phones, microwaves... pretty much any electronic device can cause interference. But since your internet connection is most likely nowhere near 150 Mbps or 300 Mbps it shouldn't affect your internet speed which is much slower.