However for LGA 1155 I prefer ASUS & Gigabyte. The non-P67/Z68 typically limit the RAM to DDR3-1333, but on P67/Z68 you have the option of higher RAM Frequencies.
For $30 IMO get the i5-2500K!
pls answer relavant to the question ...will the speed of computer increase or decrease
It's a more complex question than you or most truly realize. It depends on the MOBO, I mentioned the RAM limitations but some non-P67/Z68 can limit the Turbo-Boost but some Z68 have slower USB and SATA speeds in comparisons to other LGA 1155 MOBOs.
So the only answer is 'Yes, No and Maybe' depending on which speeds, on e.g. 'Turbo' non-K SB CPU alone then even the non-K will benefit from P67/Z68.
Here's a good video that explains some chipset differences: