Ubuntu 14.04 LTS is laggy at X450JB

Hi, ubuntu fren

Im a newcomer from window 10, waa migrate to Ubuntu.
i already use ubuntu almost 2 -3 months.
but currently i have a little problem.
hope people from here can help me.

These are my laptop specification:
ASUS X450JB
CPU : Intel® Core™ i7-4720HQ Processor (2.6 GHz, 6M Cache)
VGA: NVIDIA Geforce GT 940M
RAM: 4 GB
HD: 1 TB

These are my problem:

1. NVIDIA driver, geforce GT 940M
currently already have installed the driver, but it seems the NVIDIA VGA is not use by the Ubuntu,
im saying this because when im open the nvidia x server setting, it doesnt display information about my nvidia specification

2. My laptop is sometimes so buggy
like opening nautilus file manager could take a while to load,
open chrome also like that.
from what i know linux is lighter than window, but window seem faster load than linux.
then my application also random crash, or "ubuntu has experienced inteal error"

maybe someone has solution,
thank before, sorry for my bad english, not a native english speaker.