Apart from the internet service provider issues, these are the various reasons why your internet connection might be slow.

1. Wireless Signal Interference
Wi-Fi and other types of wireless connections may perform poorly due to signal interference, which requires computers to continually resend messages to overcome signal issues. Household appliances and even your neighbors' wireless networks can interfere with your computers. To avoid slow Internet connections due to signal interference, reposition your router for better performance and change your Wi-Fi channel number.
2. Internet Worms
An Internet worm is a malicious software program that spreads through computer networks. If any of your computers are infected by an Internet worm, they may begin spontaneously generating network traffic without your knowledge, causing your Internet connection to appear slow. Run antivirus software regularly to diagnose and remove these worms from your computers.
3. Running Background Applications
Some software applications you install on a computer run in the background, quietly consuming network resources. Unlike worms, these are programs designed to do useful work. Peer to peer (P2P) programs in particular can heavily utilize your network and cause connections to appear slow. It's easy to forget these applications are running. Always check computers for any programs running in the background when troubleshooting a slow network.
4. Faulty Network Equipment
When routers, modems or cables fail, they typically won't support connections. Certain technical glitches in network equipment, however, adversely affect performance even though connections are maintained. To troubleshoot potentially faulty equipment, temporarily re-arrange and re-configure your gear while experimenting with different configurations. Try bypassing the router, swapping cables and changing network adapters to isolate the slow performance to a specific component of the system.
5. Slow computers
When the computer you are using itself is slow then you cannot expect magic in the speed of the connection.

Please if you know any other reasons apart from the above, please share so that nairalanders can benefit.