SUMMARY If two okular processes are started within about 0.01sec of one another, then there is a delay of 30-60 sec before either okular window appears on the screen. STEPS TO REPRODUCE Enter the following command (eg. in bash), okular f1.pdf & okular f2.pdf & where f1.pdf and f2.pdf are anything (eg. a single blank page). OBSERVED RESULT The corresponding windows for the two processes open (appear on the screen) about 30--60 seconds after the processes are started. EXPECTED RESULT Both windows for both processes should appear "instantaneously". The following command, which includes a short delay between starting the two okular processes, works as expected: > okular f1.pdf & sleep 0.1; okular f2.pdf & that is, the window containing f1.pdf opens instantly, and the window containing f2.pdf opens 0.1 sec later. SOFTWARE/OS VERSIONS Linux/KDE Plasma. Okular Version 0.26.1 (up-to-date Debian-9 stretch) KDE Platform Version 4.14.26
I cannot reproduce this with the KDE in Debian Buster and Okular compiled from git. Your Okular & KDE is quite old, and you are unlikely to get fixes for that. Can you try with a newer Okular/KDE?
This is KDE4+Qt4 era software. We no longer offer support for it, sorry. I bet if you upgrade, the problem will be solved though. :)