The Pi 4 is a gigantic step up on video playback smoothness. I went ahead and tried playing a khanacademy video on my Pi4 and it's butter smooth.
You say you have a 3B+. Things to try:
- Ensure the h264ify chrome extension is running. It makes YouTube videos play smoother.
- Set GPU memory to 128
- Enabling FKMS (see below)
To enable Fake KMS:
- Open a terminal.
- Type and press Enter.
- Press down arrow key until Advanced Options is selected, then press Enter.
- Navigate to GL Driver, and press Enter.
- Navigate to GL (Fake KMS), and press Enter.
- There you go!
- Exit raspi-config, and while you're doing so, it should prompt you to reboot.
After making those above changes, my Pi3 played YouTube smoothly.
Still not working for you? Just download the youtube videos and play them with VLC or omxplayer. Or get a 2GB Pi4.
Want to run Zoom on your RPi? Easiest way is with Pi-Apps - just click Install.
Pi-Apps also includes the full version of Minecraft, Chromium Media Edition, Pi Power Tools, an automated email checker, and 16 other useful programs not in the repos.