I support the notion that the big DEs (Gnome, Plasma) are too… big. Or let’s say fat.
But my take is rather: They need to go on a diet.
I mean honestly… what can they do that I couldn’t do on my Windows 95 machine with 2MB of video memory and whopping 16MB of RAM? (Yes, MB as in MegaBytes)
16MB, because i was cooler than those who only had 8MB 
Sure, we now have Unicode and transparency and stuff, but… It’s so bloated.
Btw, I could watch videos back then without lag. (Yes, they only had ~15 frames/s at 320x240 but still…
There is absolutely no connection between those. You can watch lag-less video with 1GB RAM and 1MB of free diskspace.
The most obvious things that might hit performance would be other processes eating CPU power. Even though I guess you could compile a kernel in the background without any frame drops.
Other things that can have an impact:
- graphics driver
- desktop backend (Xorg / Wayland / compositor)
- player
Try different players, vlc for example. If the player is not well optimized, all the RAM and CPU in the world will not change it.
Now, if the video is in the browser, try a different one. Bad multimedia performance was the key factor why I switched from PureBrowser to Brave on PureOS.