Is it possible to install Stock Android? I have a Lenovo K3 Note and it comes with Android Lollipop with some custom skin and apps from Lenovo. The problem is, in spite of having a decent hardware, the phone lags too much. 

I was wondering if installing Stock Android would somehow make it better. So can I get stock Android image and can I format my phone with it?
This page has a list of custom ROMS (some based on the stock ROM) for your phone. 
Since each phone has a different hardware configuration, stock Android has to be modified to suit each phone's requirements. So Google's ROM will not work directly on your device; someone will have had to mod it themselves, and such ROMs may have other issues. Some custom ROMs have been tweaked to have even better performance than stock Android, so keep that in mind. 

If you want to minimize the chances of having other problems (connectivity issues, battery issues et cetera) on your phone, your best choice is to research more on each ROM, and flash one made by an experienced dev.

You can Google the ROM + your phone name and find out more about each ROM and how well it works on your phone.
Please note that stock ROM, refers to the ROM your phone came with, and not Stock Android. 

If you don't find any stock ROMs to your liking, consider flashing a well known ROM, like CyanogenMod on your phone.

Here is a tutorial on flashing CyanogenMod on your phone.
Read the video description first to install the ClockWorkMod ROM manager first.

Flashing any ROM on your phone voids your warranty, and may cause your phone to have more problems than you did on your phone's original ROM. Keep that in mind before you do any of this.