Here is a picture of what I see after I right click the file (I`m on audio--> panes).
Did you post the correct image? Because that image is showing a Tooltip, not the right-click menu. The Tooltip appears when you hover the mouse over a file, and can take a little while to display, particularly if you have just clicked on the file. Still only one second on my Workstation.
I have all my lossless tracks (35K) in a single flat folder
That will do it. If Windows decides to open the folder when displaying either the Tooltip or right-click menu. But MC shouldn't need to open the folder to do either of those, as the data is coming from the Library, not the folder on the disk. Or at least, I think it should be coming just from the Library. After all, that is the whole idea of having a Library that indexes all your media files.
I wonder if we actually read the folders index
It would be strange if MC did, but it may trigger Windows to "check" the folder.
Is there a recommended way to structure libraries?
The usual recommendation is: \Artist\Album\Tracks
In MC speak, that really means the expression \[Album Artist (auto)]\[Album]\ should be used in the Rename, Move, & Copy Files function, to move all tracks for an Album to one subdirectory, together. You need to use the [Album Artist (auto)] field as the [Artist] field can have more than one value, and tracks within an Album may have different Artists.