Q: why compile new kernel has all sort of problem? and it takes lots of time.
A: 
i am thinking to do this to solve:
 1. build world only once from GENERIC
 2. all others using -w to skip build world to goto build kernel directly (30mins)

--ref
use GENERIC to build world and build kernel and others
then all use -w to skip build world 

i suspect build world everytime with a new KERNEL config will mess up swap
so just keep the original build world copy in swap, and do build kernel for all other Kernel config