Iām so sorry I missed this (and your other thread, which Iāll dig back into momentarily!). I was out sick for a bit and they completely fell through the cracksā¦ my apologies there!
To clarify this situation, when you specify a remote URL, we do not actually fetch it from that remote URL directly to our servers. The JS client actually downloads it to a temp file on your own computer, and then reuploads it DatoCMS over your connection. So the ātransferā speed will be bottlenecked by your home internetās upload speed. I know this is not ideal 
I was surprised when I first discovered that too.
I think @nrothās feature request is a good one: Native Upload from Image URL Support
In the meantime, I think the only way you could speed it up is by using some other server (or maybe a serverless function) in the middle, with its own fast internet connection, that will download a remote image, cache it locally (in memory or on disk) and then reupload it to Dato. Yeah, itās a pain, sorry about that I would also like to see a server-to-server fetch where you can just provide a URL and our servers grab it directly. That would be much simpler for all involved. I hope that feature request gets some traction!