Henry,
Could be several reasons.
- Providers advertise their internet speed as "up to" a certain number of megabits per second with some fine print that states, "those speeds are not guaranteed"...
- You exceeded your data cap (on some ISPs/plans)
- Weak Wi-fi signal
- malware on your system
- upload vs download speeds; some plans offer less upload than download bandwidth
You might try restarting your router, connecting directly to the router with an ethernet cable to see what speeds you get directly, updating your router's firmware, changing wi-fi channels, etc.
I'm sure others can provide a few more things to check. I know it can be frustrating.
Richard