Gentoo File Manager is a dual-pane file explorer that is lean and fast. It has nothing to do with the Gentoo distribution. It is written in C and GTK.
Installing it is easy:
$ sudo apt install gentoo
I found this file manager to be very fast and highly configurable. However, it does not have a side pane with tree view, which I love to have.
Tried with: Gentoo 0.9.13 and Ubuntu 14.04