Red in the CPU usage bar in htop means that the time is being spent in the kernel, not that the time is being spent in “kernel threads”. This can mean that you’re spending a lot of time waiting on IO, that you’re spending a lot of time allocating memory, or similar activities that require some work to be done inside of the kernel.
Seeing as your processes are using 25GiB of memory each, I’d wager your code allocates a lot.