Long delay for second Excel worksheet to open over network

Small company network with 1GB Ethernet and i5/i7 machines, all updated and running fine. All Win 10, locally installed Office 2021, 16BG RAM.

If any of the users has an Excel workbook open and opens a second one from the local company storage (very fast M2 SSD), it can take up to 30 seconds to show up. Each time. If the same file is called from Excel thru the Recent list, it opens fast, proving that all elements are capable of supporting a quick open. Same problem for all users, all machines. Excel files are all "simple" files, some 100kB, no links to other files or whatever could slow down. *) The problem is ONLY with Excel *) File storage host and receiving computer show neither processor load nor network traffic.

Any help is much appreciated, thank you.