Thanks for your report.
I have sent you an email with a debug version of WinSCP to the address you have used to register on this forum.
Using 5.21.5
When browsing or opening folders on either the remote or local window the menu bar at the top of the interface blanks. All the command buttons, navigation buttons, everything other than "New Seesion" disappears.
The system remains frozen like this for several (10-15) seconds. If I click around I will get a WinSCP not responding message, but after the time period it will respond again and all the buttons reappear.
This will repeat for another folder when scrolling through the directories.
I have emptied the cache and that doesn't change anything. Both cache settings and the permanent cache are selected in advanced site settings. See attached image.