Ilias, yes, but the Filechooser in gtk2 is also slow. Though not as slow as in gtk3.
Every time you open a new Editor in METM and also if you open ‘Preferences’ some filechooser instances are created.
In SETM opening an image should be much faster. It must have something in common with the file system structure. Are there network folders or something like that in file system structure of Win10 per default?