I assume you mean when you open File explorer there is a green progress bar showing in the address bar?. It is running a check on the contents of the files in explorer, it might find a problem and takes its time doing a check.
Read the following thread which is a bit dated but has a few suggestions for you!. It is for Win 8 but will apply to Win 10 too.
I am seeing a green progress bar when browsing certain folders in Windows Explorer. The folder in question may have only 2-3 files, but the progress bar can take many minutes to complete scanning the
As Wolfie said but it's also part of the Indexing of the contents of a drive or a folder and can involve scanning for infections such as virus or malware.
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