GPU clock helps to speed up cycles?

Ok my question is simple this time,
does the gpu clock in a graphic card improve performance in cycles?
or it doesn’t really matter?

Yes.
However, more important than the clock speed is the architecture behind it. For example: a GTX 580 (Fermi architecture) which has been overclocked will run faster than a GTX 580 running at the reference clock speed. Similarly, an over clocked GTX 680 (Kepler architecture) will outperform a standard one. However, you would have to overclock a Kepler based card a lot to have it compete with a Fermi based one.