Does multithreading give acceleration ?

Question

I am trying to write a simple programm (Delphi) that will launch several threads and each of them will perform complicated calculations in a FOR cycle. The first from 0 to 500 000, the second from 500 001 to 1 000 000 and so forth.
I see that several threads take MORE time than a programm without threads! Why so ?
Why the system does't give my programm more processor time ?
I can launch several copies of the programm without threads and will have what I want but...
Have you ever tried to petform parallel calculations ? Where to dig ?
Thank you)

Answer 1

I have found out that NO. Modern processors or OSs organize fastest perfomance themselves and launching several threads will slow running down.