Had the same problem also on a Mac.
I was using the Download Manager for install and it was taking hours.
Decided to try The SYNCHRON INSTALLER.
Problem solved about 15 minutes per install.
EDIT: I negelcted to mention that I also used different SSD's while switching to The Synchrpn Library Installer. After further tests It appears the original SSD refused to cooperate with Synchron.... Not sure why.
So Crucial Brand SSD's worked fine as always.
Hope this helps...
Mike B.