Selection delay

When I select a clip in a timeline, a delay of 2-3 seconds occurs. Playback and the cursor freezes. But the sound continues to play. Ubuntu 18.04LTS Intel® Core™ i7-2630QM, 8GB, SSD
https://youtu.be/BNKAwdAJWDI

I am not experiencing this as bad as you. On my 10 yr old Linux system, I see a short ~1 sec pause. On my modern Windows system, it is negligible. The click-to-select code is very simple, high level QML code. This suggests there may be an OpenGL performance problem on your system.
I notice you are playing with the Center Playhead option turned on and while playing. Are both of those required to reproduce the problem on your end (I did both of those in my attempt to reproduce it)?
What is your OpenGL implementation (can be found in View > Application Log…)?
Where did you get your build/package of Shotcut for Linux?

I disabled the Center Playhead and switched the Display Method at Software (Mesa). All the same…
Shotcut is from Snap Store last version.
About OpenGL performance. I started Shotcut with “env dri_prime=1” to turn on my hybrid AMD card winth the same result.

> [Info   ] <Mlt::GLWidget::initializeGL> OpenGL vendor "X.Org" 
> [Info   ] <Mlt::GLWidget::initializeGL> OpenGL renderer "AMD JUNIPER (DRM 2.50.0 / 5.0.0-31-generic, LLVM 8.0.0)" 
> [Info   ] <Mlt::GLWidget::initializeGL> OpenGL threaded? false 
> [Info   ] <Mlt::GLWidget::initializeGL> OpenGL ES? false 
> [Debug  ] <Mlt::GLWidget::initializeGL> end 

ShotcutLog.txt (148.9 KB)
I also tried to run the program on another computer with Ubuntu 18.04LTS and Nvidia graphics with driver. There is also a delay, but it is less than a second.

And I tried to compare version Shortcut 18.12.23. Pause when selecting a clip for less than a second, this is acceptable for me.