I reckon increasing the RAM to 16 GB+ will help due to the shared video RAM on the iGPU.
Also, how large is the SSD? 128 GB?
If possible open activity monitor (showing RAM use) whilst playing the video. With 8GB RAM some macOS UI animations cause a transient swapping from RAM>SSD and this causes dropped frames.
Since I moved to 32 GB i didn't see this any more.
The size of the SSD may also be relevant because larger capacity SSD have a faster read/write speed (and may also have more free space to cope with the swapping if it is happening).
Here is a screen grab of the my RAM usage at steady state:
View attachment 887635
Now after initiating Launchpad>folder within Launchpad. The hump in RAM use is when I started entering/exiting Launchpad multiple times (key F4):
View attachment 887636
I don't have any swaps because I have plenty of free RAM (now), but you can see that wired RAM usage went up by more ~5 GB. Before the RAM upgrade this task led to UI of around 1 frame per second doing the same animation due to swaps to the SSD. So I suspect the same problem may be happening during video playback.