If you have copied a part and have inserted it (e.g. into “Track-3”), the active track than will have been changed, usually than its “Track-1”, the first track.
If you then insert again that copied part…, than, also, it can be that Shotcut than is busy for up to one minute with a CPU usage up to 100%.
(In my case it was a audio part in “Track-4” (an audio track), which was copied into “Track-1”, an video track.)
I returned to look into this, but I cannot reproduce it. If you using a build provided by us (or Snap or Flatpak), please retest on v19.02.28 and then provide steps. Please use the same terminology as used in the UI when noting the steps.
E.g. you have a project with several video and audio tracks…, maybe first three video, then maybe three audio tracks (up down).
The behavior occurs, if you copy e.g. a part in video track #3 and paste it into video track #2. Then the active track will have been changed into Track #1…