If you suspect your ISP is throttling your connection, this usually happens depending on what kind of data is being transmitted, such as torrent use. You can test for this. If possible connect your computer to your modem/router using an ethernet cable, this will give you consistent results, unlike WiFi. Now run a speed test. After that, open a VPN client and re-run the test. If your connection is significantly faster while using the VPN, your ISP is very likely throttling your service. This trick works because ISPs often throttle speeds when they detect certain types of traffic, such as torrenting. However, because a VPN client encrypts your data, the ISP can’t see what you’re doing.
There are of course many reasons for slower speeds other than ISP throttling, like traffic congestion at certain times, and connection issues in general.