PCManFM is the default file manager for the LXDE desktop.
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General use, and most operations, are snappy and responsive.
PCManFM is a very lightweight and simple file manager.
Pro Can open folder as root
PCManFM can open different folders as root, this way you don't have to use the terminal to move around files for which you need root permission.
Pro Uses tabs
Tabs can practically be managed just as in a browser, so you don't end up with windows open all over your desktop.
Pro Auto mounts drives
PCmanFM automatically detects and mounts available drives.
Pro Supports "quick-directory" typing
While in a PCManFM window, typing either a "~" or a "/" will automatically start typing into the location bar, allows for tab completion, and pressing Enter goes straight to the typed directory.
Pro More RAM is available for other processes, and the system uses less swap
Overall, the system becomes faster.
Pro Can use tabs instead of new windows
Use tabs to open fewer windows. New tabs automatically start in the same location.
Pro Fastest starup
PCmanFM is the fastest GUI file manager to start.
Con Uses a bit more RAM than thunar
Thunar uses less RAM but you pay in slower startup and stability.
Con Some operations are slow
Because it tries to be as lightweight as possible and tries to use very little RAM. This can unfortunately lead to it being slow sometimes.
Con Can't extend with scripts
Unlike Nautilus and Caja, this can't extend with scripts. If script extension is added it can become a really good FM.