I presume it is doing an HTTP downloads in the browser which isn’t ideal.
I also was able to download the PDF and EPUB this morning on the Mac.
Used safari; couldn’t figure out a better alternative , eg with curl (got a 400 with curl - there some sort of handshake, probably DRM protection).
(I didn’t bother with Chrome, so I can’t speak to it.)
On the Mac, the download was under 2 minutes for each file. (Xfinity broadband w/ a speediest of 551 Mbps down, 113 Mbps up.)
PDF: I let the download complete (in Safari), then did File->Save-As to put the PDF somewhere and give it a decent name. I’m setup to use
preview.app to read PDFs which works fine.
EPUB: The epub file ended up in my Downloads folder. I moved it elsewhere and renamed it. Double-clicking on the epub launches the Books app which presents the ebook quite nicely.
-jeff