This is my CPU and GPU usage

Recently I just updated my graphic card from gtx770 to gtx 1070 ti
I also updated my dead power supply from 600w to 750w.
My CPU remained the same, it is an intel core i7-4790 @ 3.60ghz
When I opened my task manager, there is nothing that could cost my cpu up to 90% usage, most programs only using 2-3 % of CPU at most. So why is it that when I'm playing game, my CPU shot up to 80-90%? I cannot get 300fps with a gtx1070ti in a game like csgo either, and when I put on maximum graphic setting in game, I'll get lag.
Is it an overheating problem?
CPU cannot handle GPU?
Or there are other causes?
Any help would be appreciate.

Recently I just updated my graphic card from gtx770 to gtx 1070 ti
I also updated my dead power supply from 600w to 750w.
My CPU remained the same, it is an intel core i7-4790 @ 3.60ghz
When I opened my task manager, there is nothing that could cost my cpu up to 90% usage, most programs only using 2-3 % of CPU at most. So why is it that when I'm playing game, my CPU shot up to 80-90%? I cannot get 300fps with a gtx1070ti in a game like csgo either, and when I put on maximum graphic setting in game, I'll get lag.
Is it an overheating problem?
CPU cannot handle GPU?
Or there are other causes?
Any help would be appreciate.