Treesize uses the Windows Explorer function to copy/move/delete files. This is slow and does not cope with long file names. Using robocopy would enable the feature to use multiple threads and cope with file names longer than 256 characters.
We meanwhile use multiple threads in our copy/move/delete/archive file operations. Please open a more specific new feature request in case you think that anything else should be improved.
Please add this as an option!
When using our copy / move / archive / rename dialogs, TreeSize does not use Windows Explorer but own code that is very efficient and deals with long paths.
When using drag&drop then indeed Windows Explorer features are used. We recommend avoiding to use drag&drop in case performance and long paths matter. Instead, use the features on the ribbon bar for the "Details" tab: manuals.jam-software.com/treesize/EN/details.html