The U model CPU most likely means a battery-life optimized laptop means
@strugglingmom could easily be running into thermal throttling... so setting laptop up with clean air flow to vents (if there are such) and maybe even an external fan (exact recommendation _if_ thermal throttling, depends on laptop model specifics)?
Combined with limited RAM? a number of us know how to dive deep into a system an optimize and under-powered system... but that isn't for the feint of heart.
But in terms of basics - Certainly checking the Operating System Auto-start items and disabling those that don't NEED to be running all the time (especially not when trying to convert video), like file sync tools, or ?? A number of Apps (M$ themselves being guilty of this, and thereby encourage others to misbehave as well) set themselves to auto-start (ex, MS Office, Edge, etc) when that isn't needed (and a bad thing on an under-powered system).