Bad optimization when using more than 1 monitor.

Good comments in the other posts, by the way. I'll add this.

Pr has ancient monitor usage practices, and besides that, gets cranky if you don't play by the unstated Rules of the app.

It selects screen space not by monitor used, but by relative pixel location from 0/0. Which is the upper left pixel of the UI main screen. I've argued for it to recognize screens as screen for years ... ah well.

Premiere expects it's "program" window, the one with the menu bar, to be on the UI main screen.

If the whole app is on a second screen, that's ok. But move the app's main window to another screen, and setup panel groups on another monitor ... unhappiness will likely result.

Premiere expects the Program monitor to be on the same panel group and screen as the Program Menu Bar.

So if you move the Program monitor to a panel group outside the one with the app's menu bar, unhappiness may result.

Premiere gets cranky if a single pixel line of a panel group on monitor X shows up on the edge of monitor Y.

And this one, you might not even see! So you have to make sure your panel groups on secondary monitors do NOT "bleed" over onto a different monitor. I do all mine with a slight border of my monitor showing around them.

The solution is always spend more money. Playback using dedicated hardware is the proper solution.
For as little as 150 dollars using the blackmagic monitor adapter. You can try the NDI Launcher virtual monitor as well if you have extra computers laying around. 

Get the best support by providing the following info. From User: Kevin-Monahan

Use the template below to provide us with information: 

• Issue - A short description of the problem. 
• Adobe Premiere Pro version number: FAQ: How to find the exact version of Premiere Pro you're using?
• Operating system - Windows or macOS, and which version of those operating systems?
• System Info: CPU, GPU, RAM, HD:

  • CPU

  • GPU and Driver Version

  • RAM

  • Hard Drive (and, if external, how the drive is connected: USB 3, Thunderbolt, etc.).

• Video format: Provide information about the type of video you're using. For example, XAVC-I 4:2:2 10-bit. If you're not sure, let us know what settings you selected on the camera. If you don't have access to the camera, sharing a sample file is useful. 
• Workflow details: Tell us any specific info about the project or sequence. For example, is it a multicam sequence?
• Steps to reproduce - (Very important!) Exact steps, in a numbered list, that are needed to reproduce the problem. 
  
• Expected result - What should have happened after the last step?
• Actual result -  What did happen after the previous step? 

Try resetting the preferences:
https://community.adobe.com/t5/premiere-pro/faq-how-to-reset-trash-preferences-in-premiere-pro/td-p/8236158?page=1

If that doesn't work, try resetting the Workspaces:
Reset a workspace
Reset the current workspace to return to its original, saved layout of panels.
1. Do one of the following:
• Click the Workspace menu icon and select Reset to Saved Layout.
• Choose Window > Workspace > Reset to Saved Layout.
from here:
https://helpx.adobe.com/premiere-pro/using/workspaces.html


If that doesn’t work, try updating or rolling back your graphics driver directly from the video card manufacturer’s site. If NVIDIA, do a clean installation of the latest Studio Driver (NOT the Game Driver).

If that doesn't work, try creating a new project and import the old one into it.