Why Lenovo Y40 mouse really sucks? It's the worst mouse i use on a laptop so far?

If you mean the touchpad (technically not a mouse, but a pointing device) the most likely answer is That's Just The Way It Is. Actually, nearly all touchpads "suck".
If you're really wanting to know how to improve it, the options are few.
You could try a firmware update. Not sure which Lenovo Y40 you have but here is the latest (9/3/14) for the Y40-70:

http://support.lenovo.com/...s/ds100287

You can search that site for your specific model to see if there's anything newer than what you have that will address your issues.

Other than that, you could also get an actual mouse. There are many (including designs specifically for Gaming) that are Optical (no rolling ball to get dirty/slow response & make less accurate) & even wireless.
Simple ones are not expensive (although fancy ones for gaming can be) & the only down side is it's something extra to carry around/store. Here are pictures of many different styles (did you know you can have multiple pointing devices at the same time?):

Final option I can think of - different laptop. But that's expensive and may result in another disappointing touchpad.
Hope this helped some - good luck!