A lot of reasonable solutions for File Explorer being slow can be found here:
https://social.technet.microsoft.com/Forums/en-US/26147306-64c5-466d-a211-5a6a5f012575/why-is-windows-10-file-explorer-is-unaccepably-slow?forum=win10itproapps
I had very similar problem where besides very slow opening of File Explorer in Quick Access view, I had hang-ups when creating new folders, renaming folders and deleting folders.
With the help of the good people contributing to the above TechNet post I was able to resolve my problems. The biggest problem for me was that I had two folders on remote server pinned to the Quick Access list. The remote server is not always available and when it is, it is slow - in any case this caused File Explorer to choke for up to 30 seconds from time to time. Thanks to user Jeltz191, I unpinned the offending folders from Quick Access and the problems are gone now!
Some other mentioned remedies, such as not adding recently accessed folders to Quick Access and setting File Explorer to start in This PC rather than Quick Access, are reasonable settings anyway.
The suggestions to run sfc and DISM did not help me.
I am glad I did not have to enable Cortana, as was also suggested!