The green bar is simply an indication that Explorer is busy. It is performing some task that is taking a long time and so it shows a green bar so that you can see its progress. The question is, what can it be busy doing?
There are several possibilities and this behaviour can occur when there are a lot of files in a folder, especially if they are images and Explorer is having to create thumbnail images from them. It takes time to do this and to reduce the delay when opening a folder you should reduce the number of images. Organise them into subfolders so there aren't so many to deal with at a time.
Whether you see the green progress bar depends on the view selected and a text listing avoids the need to create and display thumbnails. It is faster and so you don't see the green bar. Folder views are remembered, so if you switch to List or Details view then the next time you access the folder then that view will be used again.
The sort order can be a factor and if you have selected a sort order other than the default alphabetical listing, then it takes time to put the files into order and when there is a lot of files, it can cause the green progress bar to appear. Sorting by size, date, and other attributes is slow. Sort by name and Explorer is faster.
You will also see this sort of creeping green bar behaviour if you add a networked location to Explorer. For example, one day you might connect to a NAS (network Attached Storage) drive and use it to copy files, make a backup or whatever. Windows will remember this and the next day when you use Explorer, it will try to reconnect to the NAS drive.
This is fine if it is switched on, but if it is off then Explorer will keep retrying until it eventually gives up and realises it isn’t available. All the time it is trying to connect, it shows that green bar sliding across the top.
If you connect to a shared folder on another computer on the network, the same thing can happen. It that computer is not switched on, Explorer wastes time trying to connect to it and shows the green progress bar.
There are two possible solutions and one is to make sure that any shared resources like NAS drives, USB drives, other PCs with shared folders and so on, are all up and running when you use your PC. The other solution is to disconnect from the shared resource. Right click the drive in the Explorer window and delete it. The next time you open Explorer it won’t look for the resource. If you need it, it can easily be added again.
It is a bit of a nuisance having to add and delete shared resources though.
Shared drives that are not available are not the only cause of the green bar. Searches can be saved and when the Explorer window is opened, the search may be run again. This takes time to complete and so a green progress bar is displayed. There isn’t much you can do about a search except to delete it if you don’t need it. Make sure the search box in the top right corner is clear.
Update: The solution
The above tips helped a bit and reduced the green bar a little, but what really got rid of it was changing antivirus software. Windows Defender seems to be quite slow at scanning files and when opening folders it causes the green bar to slowly crawl across the top of the window.
Windows Defender is not the only slow antivirus program and there are others. See Does antivirus software slow down the PC?
Installing Avira security, even the free version, will cure the problem. The green bar has now gone away