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Contact your Internet Service Provider (ISP) or operator
Maybe the reason why your Internet connection is slow is the ISP/Operator itself, due to technical difficulties they are not aware of. Contact them and let them know about this issue and they will instruct you on how to troubleshoot slow internet connection, and resolve the issue by yourself, if possible.
Check your Internet (mobile data) traffic
If you use mobile data and you have the "slow internet speed" issue, you might have exceeded the limit you are subscribed to. Every application you have installed on your phone is running in the background and using your Internet when your Wi-Fi or mobile data is on, unless you have instructed your device otherwise (i.e. blocked certain apps from using the Internet speed).
To check your internet traffic:
Go to Settings and tap on Data usage. From there you can see how much traffic you have used.
Open the Settings app on your iPhone and tap on Cellular. Scroll down and you will see an amount of data displayed for the Current Period, which doesn't automatically reset every month, so the data usage you see displayed here may be a total from many months.