It can slow down for many reasons. You may be out of data. There may be inclement weather negatively affecting your gateway. Your WiFi signal may be weak (if you're using WiFi). You may have multiple devices using the net at the same time. It may be due to heavy congestion, as well. There are many possibilities.
If you would like to address your speed issue, please start a new topic in Tech Support, which you can do here.
AMD FX-6100 | Samsung 250GB 840 EVO SSD | Western Digital Blue 500GB HDD | 16GB DDR3-1866 | EVGA Geforce GTX 550ti | Windows 10 Pro 64-bit