The linked video is a pretty slow entry to the genre. Look at e.g. https://www.youtube.com/watch?v=TgmTsa3rFU0

There's approximately three animated memes per second in these and people are usually using pirated Vegas to make them.

They also had to pick a one-second slice of a "thematically-related" music track to set each of the ~100ish readings to. That would take a fair amount of time during editing, no?

Those are called sound f/x. They are quite common and are litterally just 1 second sounds to be dropped into a timeline. There are libraries upon libraries of them available. They used to come on vinyl and had to be recorded in to something useable. Then they came on CDs and could just be ripped. Now, they float around as YT videos or other websites royalty free.

This would be common knowledge amongst people that have spent any time in an edit bay as a client or as an editor. It's one of those things that those that know just sit back and smile while those that don't are in awe.

It's depend how you are skilled and how good your editor software is. With bad software it can take ages.

While I may not be a 10x dev, I'm at least an 8.5x editor. Experience goes a long way as well. Knowing when/where to use certain technique, how/why something takes longer, etc all add up to make an edit session go faster.