Lossless Video Manipulation
Here are some tools that can be used to manipulate videos losslessly (and hence very quickly).
LosslessCut
LosslessCut allows you to rearrange videos at keyframes or cut portions out. It has a very simple and easy to use GUI.
https://github.com/mifi/lossless-cut
Xmedia Recode
Xmedia Recode doesn't have the prettiest interface, but it does have many many options, including a "copy video" and "copy audio". In particular, it has the ability to "copy audio" while also normalising the audio. You can set the audio gain manually, or get the program to automatically determine what the audio level should be. This function is very very useful, when recording your own videos, to do a simple audio level increase without having to re-encode the entire video.
https://www.xmedia-recode.de/en/help/afilter/afilter_volume.php
My MP4BOX GUI
MP4Box GUI provides a GUI interface to MP4Box. It allows you to easily Mux, Demux, Join, or split MP4 files.
https://www.videohelp.com/software/My-MP4Box-GUI
VirtualDub and VirtualDub2
VirtualDub supports a feature they call "Smart Rendering". Smart rendering identifies which portions of a video stream must be re-encoded and which can be copied directly from the source file. This can save a lot of time, as typically only the regions around an edit must be re-encoded. It also preserves the quality of most of the non-edited video.