Why Kali Linux Running Slow?

I installed Kali Linux latest version from official website last week as a dual boot alongside windows 10 without problems, but its been running slow with delay response and lags all this time specially when using Firefox (5 tabs was on).

I am using Lenovo Y700, 250 GB SDD (where the OS's been installed), 1 TB HDD, 16 GB RAM, 4 GB nvedia geforce GTX 960m.

ps. i noticed the problem today when i was trying to install Tor Browser and followed a tutorial and modified "start-tor-browser" by changing value from 0 to 1 in this line {if "`id -u`" -eq 1 ; then}

*if "`id -u`" -eq 1 ; then

complain "The Tor Browser Bundle should not be run as root. Exiting."

exit 1*

is there anyway to help? thanks in advance and sorry for my English.