Navigating a share is fine. Opening a file is slow.  

Running Windows 10 Pro 1903. We have five systems, all running the same version of Windows. One acts as a 'data server' and has three internal drives. Drive 1 is for boot/system purposes only. The other two are used for data storage only and are shared as Data1 (L:) and Data2 (M:).

From my PC (and all others), I can navigate L: and M: fine in File Explorer - pop in and out of directories with no delay. But if I double-click on a PDF file, it will take about 20 seconds to open the file. Once Adobe Reader is up and running, however, double-clicking on subsequent files opens them immediately (about 1 second). This same behavior is seen in other apps (like Word) in which the initial file takes a long time to open, but once it is opened, other files open up right away. Once the app (Word, Reader, etc) is closed, the delay shows up again when I try to open something new. This also happens when simply starting a clean session of Word: Slow for the first doc. Fine for others after that. (Note: There is NO delay in opening a split Access database - the front end of which is stored on the C: drive.)

(Additional information: Copying a very small file (16kb) from a shared drive to my desktop introduces a pause - 10 seconds or so - at 99% progress before it finishes the task. Then, additional files (125MB) drag over right away - as in about 1-2 seconds. I waited about 15 minutes and tried it again and found the delay with the first file dragged again.

Yesterday, I disconnected from all shares and powered down my system. On the 'server', I created a new user, went into the sharing permissions, and added that new user with full control. I then powered up my system and mapped the drives to the shares, logging in with those new credentials. Everything worked as it should at that point: Initial opening of files was as fast as it should be. That was at 2pm yesterday. Today, everthing is fine until about 10am when the delay suddenly reappeared. And that's where I am now.

Anything obvious jumping out that I should look at? At one point - maybe a month ago - we weren't having these delays.

Thanks in advance,
John