This is an expected behaviour as you are loading 1:1 image previews and that takes time to load for the first time.
Please refer https://helpx-internal.corp.adobe.com/content/help/en/lightroom/kb/optimize-performance-li ghtroom.html and let us know if that helps optimizing Lightroom.
Can you tell us some more about:
- Your version of LR
- Your computer (OS, Hardware specs.)
- The format of the files in question
Akash - thanks so much for this. Life saver!!
Your system is loaded down with other processes taking up CPU time. Shut down those other processes while working in LR.
Take a look at either Activity Monitor, Mac, or the Task Manager, Windows, to see what else is running on your system when this happens.