Issue with long delay opening pdf files

Hello,

our customers/users complain about PDFs not showing up or having a massive delay if they were opened through our Software.

Some of them send me a screenshot of the known DDE-Connect error that happens sometimes.

As software developer i first checked our software and how we open the PDFs.
All steps from creation to deployment to execute the open command are almost instant and successful.

We use this windows shell function to open the PDFs (.. surely as everyone does):
         ShellExecute(handle, "open", <fully_qualified_path_to_folder>, NULL, NULL, SW_SHOWNORMAL);
After this command we are out of scope what is happening...

We adviced them to update (cleanup/reinstall) to the latest version of Acrobat Reader.
.. and to do a lot of stuff / hotfixes we found in this forum (cleanup temp folder, registry-fix)
... besides that the most are too complex for a simple windows user
!! we don't gave the hint to disable protected mode .. but tested it on our own

None of them seems to fix the issue...
And for us it was hard to reproduce the issue but well we have those delays on several machines with different Reader versions too.
These delays are not that much but they are mainly about 5-10 sec - sometimes more - and makes our software unusable.
Maybe the main process is in a busy / hangup state but we can't clearly say that and that can't be or our users don't accept this as a reason.
It happens randomly .. maybe at some circumstances - ie. shortly after opening one pdf a second pdf has a delay .. but not always.

So i did some more intensive testing to reproduce this issue so i could create this post and give you a hint.
And i found one that is 100% reproducible with double click open at Windows Explorer that shows the exact same result as we can see if the delay happens when we open pdf by our software.

For this i used Reader DC v2021.007.20091 and Microsoft Windows [Version 10.0.18362.1256] - even in safe mode .. so no security software is involved..
1. Open Explorer and navigate to a folder where 2 pdf docs are in there
2. Open one pdf doc
3. Resize the explorer window some smaller and move it to the bottom right so you may access the window even when acrobat Reader is opened / pops up
4. Make sure no PDF doc is opened
5. keep double clicking alternately on those two pdfs
>> if one is already opened - the window will get focussed
>> so there should be always be a switch between the windows
7. after a few seconds the focus switch between those two will stop
>> in task manager you see a lot of background tasks and after a delay they will be processed and you may see a lot of switches in a few ms

Could you follow me up and reproduce this one too?
This may be just "a first start issue" but this is how our users work.

Any solution or hint on this?