parallel for loops not working

Hi.

I'm learning multi - thread programing. For start I use "parallel for loops" and I was suprised that is not working. One -thread loop work few times faster (depends on settings) than multi - thread. I don't know why and this is my ask to correct my Vi to work properly.

Computer Lenovo, G580, Windows 7 home, 64 bits

Intel Core i7 3632QM
Ivy Bridge
Specification Intel(R) Core(TM) i7-3632QM CPU @ 2.20GHz
Package (platform ID) Socket 988B rPGA (0x4)
Number of cores 4
Number of threads 8

LabView 2011.