Finally this issue was solved!

For various reasons my client decided to move the MS SQL-server 
in-house. I was hoping that this would make the problem go away, but it 
got even worse!

Instead of “only” slow performance we got dropped connections. We could 
open the database, do inserts but not select! Still we had the the 
strange problem that only one shop was affected, others were OK.

My client’s ISP assured us that everything was OK with the firewall, 
hardware, wiring. They inspected packages, ping results – “everything” 
but no fault were found. Of course, they blamed me, Paradox and 
Microsoft for the problems.

The problematic PC is acting as a POS which means it is only used for 
this specific purpose. During my testing however, I also did some other 
operations such as file transfers and web surfing.

I then realized that this traffic was also affected! Opening websites 
took abnormal long time. Copying files from the PC to a server worked 
fine but from server to PC never worked.

Armed with that fact I could push the ISP again and finally they 
“rebuilt the LAN-configuration with another standard” for the 
problematic shop and then everything went back to normal!!

How many days and hours I spent troubleshooting this problem I don’t 
dare to think about, but it was good to know that good old Paradox was 
not to blame!