My next question was going to be about the files you have on your shares.
Explorer will as you say, go through and verify all files in a zip file.
I have not seen this issue as I don't have any zip files in shares exported to my VM.
I will now replicate this and see if there is anything we can do to speed up the scanning of files contained in zip files.
It might help or might not but it is worth looking into this.
I will also see what occurs when I use the Microsoft CIFS share too.
Thanks again for your help in tracking this issue down.