For some reason you can sometimes have this white bar appear on Windows File Explorer. Fortunately it is an easy fix. Launch Internet Explorer, Yes IE, some of the configuration settings affect both Internet Explorer and Windows File Explorer.
Right click on the Title bar and check the box on Lock the toolbars.
Close any open File Explorer windows and relaunch it. It should now be gone.
It looks like the Windows Defender Real-time protection can cause issues when trying to copy files on a local drive even if the drive is an SSD. Typically a copy seems to start out great, but then slows down to a crawl less then 1MB/s copy rate. Probably has to do with Defender having to scan every file as it is copied.
To fix/increase the copy speeds you can turn off Real-time protection.
To turn off Windows Defender Real-time protection, open up Settings > Update and Security > Windows Security > Virus & threat protection “That should open up the Windows Defender Security Center” > Virus & threat protection settings