To my understanding of the muticore checkbox… the increased cpu usage is overhead required to make the multicore processing work.
It is more efficient without multicore but you need more processing power. You may only be at 15% usage but that is overall, what are the usages per core? If one core hits 100% you’ll get dropouts. You may not be able to see the core hitting 100% with a monitor as the refresh rate might be too slow and it may not visually show up.
On the i9 9900k that I use with Ubuntu (8 core, 16 thread), with most filters 2 cores do a majority of the work, the rest have low usage… by majority I mean between 50 and 85% usage on those 2 cores.