A lot of these are not so much about saving 100 hours, rather being faster when it does matter, as well as doing something interesting (automating a task by writing a script) instead of doing the same thing over and over again. This is also why you shouldn't take
https://xkcd.com/1205/ too literally, although it's good to keep in mind.
* frequent (automatic) backups (saves time and mood when things go wrong)
* i3
* fd (alternative to find)
* ripgrep
* keynav (don't use your mouse)
* !bangs on duckduckgo
* calcurse (terminal calendar)
* readline: C-h C-b C-a C-k M-b M-f C-u etc.
* howdoi (get SO answers in the terminal), especially useful for things you somewhat know but just forgot the syntax of.
* youtube-dl as already mentioned somewhere: you get out of youtube ASAP to prevent their algorithms from tricking into staying there. I also use it to watch videos later when I don't have internet, using a syncthing folder on my mobile which only syncs up to 10% remaining space (so that it doesn't fill with 100s of videos, just a few, enough for some trip). Videos are also rather inefficient in terms of communicating ideas, so better to keep them for when I really don't have better to do.
* in the same vein, adapt the speed of (technical) videos you watch, to skip the fluff and focus (even rewind) the difficult parts. Skip all ads, everywhere.
* uBlock Origin, for the same kind of reason: just block any annoying, time-consuming parts of website. If I need to go on some website regularly, and they happen to have a news section I don't care about (like their twitter feed), I just block it to get it out of my way.
* even use a text-only browser (I currently use w3m, which could show images in theory) for things like HN. HN itself works well, and its good links as well. You just trim a lot of the fluff, ads, etc.
* an easy way to sync files between your mobile and your computer (I use a combination of "Notes to Self" in Signal with an auto-destruct of 1 week for temporary stuff so that I don't have to delete it manually, and syncthing for longer-term things)
* script your way out of any repetitive task: if you need to register periodically to something, either use their API if they have one, or use selenium to automate it. I was really surprised how little time it took me to write my own script and learn how to use selenium.
* in general, rely less on proprietary software: it has the potential to break ("introduce new and shiny features and somehow make some old ones disappear" or just change the pricing-model) more regularly, and you'll have to switch which can be a burden. If some free (libre) software breaks hard for a lot of people, chances are that a fork will happen and the transition will be easier. This also applies to SaaS.
* try to avoid desktop apps: they don't compose well, you cannot easily script them compared to a CLI. It's also better than having a full remote desktop when you resort to SSH (especially on bad connections).
* try to learn the default keyboard shortcuts of software you use, unless they're really crazy: less config (which you'd have to sync and maintain), easier to use a computer which is not yours if need be
* regularly check that the commands you write in your terminal are not too verbose (use aliases / functions): https://news.ycombinator.com/item?id=22853646
* check what's possible with frameworks like https://github.com/sorin-ionescu/prezto
* some dotfile repository to get you started in 5min on new computers with the same setup you're accustomed to.
* mutt (mostly to easily write emails in vim and grep emails from the terminal)
* cronjobs and reminders (I get new music albums on my phone regularly from my music library, so that I don't have to choose them manually (it's my library so I know I like them anyway). I used to actually spend time choosing music from my own library.)
* typing from your phone is inefficient, avoid it if possible. Batch your (non-time-critical) messaging. Most modern messaging apps have a desktop version these days.