The issue is trying to run a video in Firefox. Modern web browsers consume a lot of resources. Also they don't use your hardware efficiently for video playing. You need to take some time to set up a native video player application to play YouTube videos. This better uses the SBC hardware acceleration without wasting precious resources.
How to play YouTube through SMPlayer
Use your operating systems software installer to Install the latest versions of smplayer, smtube, and mpv. Use smtube to select a YouTube video. This sends the network stream URL to smplayer which detects its a YouTube video and downloads the latest yt-dlp to help stream it. If everything is up to date, it plays great.
Not all OS keep their software up to date. Some prefer older stable packages. So its important to use a OS that keeps this software updated. I know for sure MX Linux works with its default software repos out of the box. Its available for Pis, though I have not personally installed on a PI.
Configure SMTube To Use Invidious
Once you get YouTube videos playing, go into settings of smtube to change the web page from tonvid to a custom invidious instance. Pick one thats ideally from your country and that lets you register an account. That way you can import subscriptions and personalize stuff.
Hiccups when using smtube to load an invidious site: the default language will be some foreign language. Make sure you know how to go to settings in invidious and change to english.
To load the video click on small youtube icon bottom right of video.
Old Hardware Given New Life
I have revived lots of old PCs over the years. Giving them a new lease on life with up to date linux operating systems for friends and family. I have a 15 year old laptop that was finally having a hard time running latest linux mint xfce. This week I got to work reviving it.
I gave mx linux a shot as I liked ExplainingComputers review of the OS and thought it good fit for my use case. Installing these programs right from MX's software repositiories was a breeze. Youtube played effortlessly! MX is pretty minimal and im sure most pis can run it okay, so give it a shot if you want a OS with up to date repos for these packages if youtube is one of your main concerns.
When you 'stream a video' from firefox it just downloads the video in small chunks at a time instead of the whole thing at once. These chunks of downloaded video are saved to temporary memory called a 'cache' and deleted after you are done the video.
Yes yt-dlp is most often used to download the entire video as a digital file onto permanent memory; however it doesn't have to be used that way. Other applications like smplayer and mpv can work with yt-dlp. Using it as a component to do the heavy lifting of talking to youtubes servers and streaming video in the same exact way firefox does.
Doing a quick search, there are some projects to implement mpv with sponsorblock. Im not the most technical person and prefer not to get my hands dirty with complex hacked together scripts that require compilation or whatever. Thats not to discourage you if you want to follow up on those things know people are working on it but if you aren't a power user it may be a hard time to get that kind of thing working.
Open vlc's playlist viewer with ctrl+L or right click > view > playlist
You should see a bunch of IPTV streams to choose from. Go to search bar located top right to search playlist for the stream you want. You can look for a more specific iptv-org playlist for your language and stuff. when commercial breaks happen it just shows a still frame. If nothing is playing right away try waiting a few minutes.
I hope all this has helped you out, Lumisal. I updated the formatting on my comments to closer resemble a guide in case you decide to link them in the future. Enjoy your open and energy efficient computing!