Threading vs single thread

Question

Is it always guaranteed that a multi-threaded application would run faster than a single threaded application?

I have two threads that populates data from a data so

Answer 1

No, it is not. Because when you do multi-threading, your CPU has to switch between thread, memory, register, and that costs. There are some tasks which are divisible like merge sort, but there are some tasks may not be divisible to sub tasks like checking if a number is a prime or not (it is just my sudden example), and then if you try to separate it out, it just runs like a single thread problem.