I can speak from experience, idling when my 4600+ was new on Windows XP would eat up around 30% of the CPU resources. Windows 10 background processes probably consume even more, as it was designed around CPUs a decade newer. Like mentioned above, the old video card might be causing you some problems. I used to have a 3 GHz Pentium 4, 1 GB DDR-333, and FX 5200 based system and watching youtube above 480p was hard for it to process, and anything lower had usually worse sound quality than the newer athlon 64 x2 based one and more stuttering. It did beat the crap out of the CPU and RAM but I think newer video processing standards the card wasn't made to do is your problem.
If you got a flash drive, humor me and try something like Linux...