[Solved] Very low fps (5-25 fps) in all games ! Computer nearly unusable (for gaming)

Hello guys,

After 2weeks of search and and lot of different tests I decided to ask specifically about my issue because nothing that i tired worked out until now..

I have (I think) a pretty decent gaming setup, but since 2 weeks I'm not able to play any game because I have very low fps on all of them.
Some examples:
- Diablo 3 : 20-30 fps idle, below 15fps in action
- Overwatch: 15-25 fps idle, 5-15 fps in action
- Warframe: same as overwatch
- league of legend: 50-90fps idle, 10-20 fps in action
- Heroes of the storm : same as overwatch

To put a bit more context, here are the specs of my setup when everything started:
Motherboard: MSI Z170A Gaming M5 (MS-7977)
CPU: Intel i5 6500
RAM : Kingstone DDR4-2400 (2x8Go)
Video Card: Zotac GTX 660Ti
Hard Drive: Samsung SSD 850 EVO 250GB
PSU: Corsair 850W
OS: Windows 10 Pro

I'm not sure 100% when it started, but I have the impression it was when I updated my GC drivers and Gforce experience.. after those update, all my games started to lag (and i even have troubles browsing sites with lot of images/videos like facebook or streaming sites, it's incredibly slow and laggy).

1 week before the lag started, I ordered a new GC. I decided to buy a new one before the problem started so it's not related to the issue.
now I have a MSI GTX 1070 X 8G, but nothing changed, issue is still exactly the same !

I tried a lot of stuff:
- Updating all my drivers (including NVidia drivers, using DDU for a clean reinstall)
- Downgrading my drivers to previous versions as suggested on some posts
- Full reinstall of windows
- Messing with Nvidia 3d settings like power options or V-sync
- Tested my RAM with the windows ramtest tool
- used 3Dmark to test my setup (but I'm not able to interpret the results)
- Stress tested my CPU, GPU and HDD
- I tried to start games from all my hard drives to check if it was not a problem with one of them
- I tested online and offline games to test if it was not network related.

Nothing in this list had even the slightest impact on the issue I have... So I'm starting to get quite desperate now and I don't know where this come from.. It's frustrating to have such an expensive setup and not being able to play even basic games..

Hope you guys can help me because I don't know what to do at this point...
Thanks in advance !

I did a few tests on overwatch and diablo:
On overwatch: my CPU runs at 100% all the time
On diablo3, my CPU runs between 65% and 100%
The max CPU temperature reached, both tests included, is 39°C

Edit: After few other tests, i'm not sure anymore.. I was using MSI Afterburner to get those info, but then I opened the task manager and there was a big difference. MSI afterburner was showing high CPU usage while the task manager of windows was not exceeding 25% of CPU usage !
What tool do you suggest me to monitor my cpu?

Yes, I was monitoring while playing, I was looking at the charts on my 2nd screen

Here is a test i just did, playing 10min to Diablo3

Each CPU core stays at 800Mhz all the time
Temp for CPU stay quite low, max temp reached is 32°C