It could be a number of things. In the case of the modem/router, you may want to reboot it anyway, as it is not uncommon for one device out of many to not function correctly until it is rebooted.
If there are many other devices connected to this same WiFi, then that itself can slow your connection. You can try connecting via Ethernet and see if you get better speeds.
I would also suggest you clear your cache, history, etc., on the browser you are using.
Lastly, try this:
1. Go to "Control Panel".
2. Nex,t go to "Network and Internet" and then "Network and Sharing".
3. Now, click on "Manage Wireless Networks".
4. On this screen, remove all wireless connections by "right clicking" on it and select "Remove network" from the drop down menu.
5. After doing this, close the open windows and reboot the computer.
6. Once the computer is fully reloaded, scan for the "WiFi networks".
7. Click on the available network that you want to access and select "connect".
8. It takes a moment, but then you will be asked to give a password for your WiFi connection. Type in the password and click the "connect" button.