lightroom loopt vast /

The first thing that you should try is to switch off the GPU support from the Lightroom preferences and check if that helps to fix the issue.

Go to Lightroom > Preferences > Performance tab > Uncheck "Use Graphics Processor" > Restart Lightroom.

https://helpx.adobe.com/lightroom-classic/kb/lightroom-gpu-faq.html#troubleshooting

https://helpx.adobe.com/lightroom-classic/kb/troubleshoot-gpu.html

If Lightroom doesn't start correctly the please see "Solution 2" in the document behind the second link.

Troubleshoot GPU issues | Lightroom Classic (adobe.com)

Another step is to try to reset the Lightroom preferences:

https://helpx.adobe.com/lightroom-classic/help/setting-preferences-lightroom.html

It's recommended to backup your preferences before you reset the preferences to the default settings: 

https://helpx.adobe.com/lightroom-classic/kb/preference-file-and-other-file-locations.html

If this doesn't help we need more informations about your environment.

Which operating system do you use? 

Is your system up-to-date? Do you have install all patches and updates for your operating system.

Do you have install the latest version for all your drivers (especially the graphic card)? 

Which version of Lightroom do you use?

Please post the exact version and not only phrases as "recent", "latest" and so on.

Do you get an error message? If yes, please post the exact message or an screenshot.

Are you referring to Lightroom Classic or Lightroom Cloud-based