I've been having this issue for so long now! For some reason, my internet is blazing fast until I install Hamachi. Certain website downloads are affected but others aren't. for instance, downloading a zip file from "downloads.oneplus.com" WITHOUT Hamachi shows 3 minutes left. AFTER installing Hamachi, that same file now says 20 Minutes.... Same issue happens with Microsoft's download servers, when trying to download the Windows 10 iso, Hamachi just DESTROYS the download speed when my internet is more than capable of downloading large files quickly!
Even uninstalling Hamachi leaves the same affect, almost like there's something left in the system that causes this, registry data maybe? Any pointers?