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Hi @LenLen1, Thank you for reaching out, and welcome to the community! I'm sorry to hear about your problem. The delay you're seeing is the time it takes for Lightroom to generate a full-quality preview on the fly because one wasn't ready beforehand. 

Could you please share more information, such as whether you see any error messages or grayed-out thumbnails? What type of images are you working with? Does this happen with specific formats or with all of them? Knowing your system configuration would also be beneficial. From Help> System Info> Copy and paste it into a text file and attach it to your response.

Adding to what Robert36972564 and Bill Sprague have suggested, you can also try these fixes and see if they work.

1) Try to deactivate the GPU support and check again.

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

Adobe Lightroom GPU Troubleshooting and FAQ

2) Check out the correct answer in this thread: Solved: Re: I cannot see thumbnails and images - Adobe Product Community - 15298694

3) Rebuild Previews: Try out the steps provided in this Helpx article: Lightroom error on preview cache or displays gray thumbnails.

You may also refer to the information in this Helpx article to Optimize Lightroom performance.

Let us know how it goes! 

Cheers!

Adobe's Terry White released a short video a few weeks ago on how to speed up Lightroom Classic.  

https://www.youtube.com/watch?v=AE64rgsKtN4

Do you store that images and the LR Catalog on SSDs? Preferably even two seperate SSDs? Lightroom needs quite fast storage for accaptable performance.

And do you know about standard and 1:1 previews in LrC already? You can generate them on import or later via menu. Otherwise they will be generated when you open an image, which might take a few seconds.

Make sure that the standard preview resolution is set at or slightly above your screen resolution in the Lightroom preferences/settings.