dell laptop inspiron 15 3567 is very slow need help on RAM upgradation

I am an IT professional. I need some help.

I am having laptop Dell inspiron 15 3567 laptop which has processor core i3, 6th generation 2GHZ and a DDR4 RAM with 2 slots and one slot is filled with 4GB DDR4 RAM 2400MHZ in one slot.

When I ran CPU-Z, it shows like single channel in Memory tab and Single Rank in SPD tab. L

aptop is very slow and taking too much of time for simple document also.

In task manager, in memory usage, only 200 MB left.

I want to add more RAM in 2nd slot to existing inbuilt 4GB ram which is in 1st slot.

Could someone please give me correct advice to make my system faster?

I am also asking some questions below:

Can I add extra 4GB ram?

OR

Can I add extra 8GB ram ?

OR

Can I remove inbuilt 4GB and replace it with 8GB ram?

Can you pls tell me CL latency, clock speed and all ?