Q-Dir Portable (64-bit)
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Full user review
Have used and tried all the well known and less known Win Explorer alternatives. It's so lightweight, I just use it whenever I want access to 4 to 20 folders at once. Doing this in windows explorer is just gonna muck your mind up.
Start-up is instantaneous no matter how many tabs in the quad setup. Is likely you'd get lag with any other app.
It's the finer things. And every build it just gets better. The author to this day amazes.