Pentium is a heavy downgrade.
To check what in the world is eating up that CPU, open Task Manager by pressing Ctrl+alt+delete, click on "CPU". See which process is taking up most CPU usage when your system is idle. Most cases, it's the super stupid ridiculous and annoying Windows Update, or a simple antivirus scan. My Windows 10 has windows driver foundation host eating 40% CPU forever, I need to terminate it manually on each boot.
Always try doing things by software before changing hardware.
If your process isn't listed in task manager, go to "performance" tab and click "start resource monitor". Then go to the "CPU" tab and click on CPU Usage.
List which process is taking up most CPU. Generally it's svchost.exe (netsvc) which is basically Windows Update doing stuff.
UPDATE: An i5 can't bottleneck your GPU that much. I myself used i5 2310 :LGA1155 with Strix 970 4GD5. It never had any bottleneck,
@Dark_lord69, formatting is the absolute last thing a user should do according to me. It can cause many stupid annoying problems. Resetting everything, installing all drivers again, issues related to windows key management, etc. Just don't think about formatting. Try defragging instead. And slow HDDs is not even related to the CPU.