Very frequently when I am downloading a file, I notice that it downloads very slowly on Safari. This occurs from just about any website I am downloading a file from. However, after a restart of the download (canceling and then restarting it), the download starts and resumes at full speed.
This has occurred to me on macOS High Sierra, and now occurs to me on macOS Mojave.
Is there any reason Safari does this? The only extension I have enabled is uBlock Origin.