Mapping Problem. Down key have a slow response time.

Dear List,

I have remapped "up-down-right-left" keyboard keys. It is like 

Vim Mappings  My Mappings

-------------- --------------

h-move left k

j-move down b

k-move up n

l-move right l

I have remapped these keys  in the .vimrc file  to work in insert mode and control mode.

"Normal Mode

noremap h b

noremap j n

" noremap l l

noremap  n k

noremap  b j

noremap  k h

"Insert Mode

imap <C-space> <esc> 

imap <C-l> <right>

imap <C-k> <left>

imap <C-n> <up>

imap <C-b> <down>

It works but in Insert mode "<C-b><down>" has a slow response time from other mappings. It is not as fast as other mappings. I wonder what I am doing wrong.

Regards,

Ozhan