My PC is not using 100% of GPU

Hi guys, I have a PC without GPU and render time was very slow.

So, I got with a friend one Geforce GTX 1060 6Gb and install it today.

The render time reduce a lot, from 15min to 6min for ex.

But the use of GPU is not in 100%, the configuration at General/Preferences is Hardware Aceleration with CUDA.

The memory in GPU was very low to, but my output file was only 650MB.

Now I need to buy a GPU to me, and return this to my friend.

With I´m not using 100%? It´s because my CPU or RAM?

If I buy an GTX 1050ti the perfomance will be the same in my case?

I´m glad with 6min of render, and don´t want to change the PC.

In my home I have anothe PC, with a core i5 8400, maybe with this CPU should be faster?

The files are in my server, and I acess them directly to work by the network.

But I noted that Premier do a buffer in my PC, and only export the final file by the LAN at the end, so, my 100Mb LAN should not be a problem.

My PC:

Intel Core i5 4460

Asus H81M-A/BR

8Gb DDR3 1666Mhz

SSD Kingston 120Gb SV300S37A120G

100Mb LAN

Thanks!