Is it not guaranteed that a threaded operation will not hog the main thread's resources?
You are running on a computer that's playing host to a preemptive multitasking Operation System. Nothing guarantees your thread resources. There is nothing you can do on a computer that will guarantee that your thread's computational resources won't get taken away. Either by your own threads or some other processes.
That's life in a preemptive multithreading environment. Best live with it. The only thing you can do is minimize the chance of something happen; you can't eliminate it.
Without knowing anything about your inter-thread communication system, or how much work you're doing in this other thread, or any of the particulars of your situation, no solution can be offered here.