most likely your machine hasn't enough oomph, or graphical hardware acceleration is unavailable, or both.
for the gpu, please post the output of
Code:
lspci -k | grep -A5 VGA
(although it's possible that the inxi ouput provides the same info).
other than that, don't use youtube directly but use mpv & youtube-dl. hey, there's even a
firefox addon!
edit:
oops, cinnamon on an old celeron... no, no. lightweight you must go. MATE is good.