What are the possible causes and solutions for low download speed despite high speedtest results?

 
I have a high-speed internet connection that usually shows up to 500 Mbps on speedtest.net, but my actual download speed is much lower, ranging from 2 MB/s to 500 kb/s. This happens regardless of whether I use wireless or ethernet, chrome or JDownloader, or different PCs. I have checked the network settings and firewall, but nothing seems to improve the situation. How can I diagnose and fix this problem? Is it related to my internet service provider or some other factors? I appreciate any expert guidance on this issue.
 
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