There is a good possibility that your ISP is throttling the speed to your IP address. Of course, they won’t admit that is what they are doing.
One way to check this is using a VPN service which will shift your IP address and frustrate the ISP’s throttling. If the speed of service improves, or noticeably changes, when running through the VPN then you’re likely being throttled by your ISP.
If you’re being subjected to throttling, usually during peak evening and weekend hours, your ISP won’t stop but, at least you’ll have a work around for a while.
Eventually, the ISP will make note of the VPN’s library of IPs coming from the account and will begin throttling them as well. The good news is that there are a lot of VPN providers out there, each with their own IPs.