I've seen that happen for many reasons. Most common is the HDD was from the old system. Windows does not do transplants well and has never dealt with drivers either. All that is known and left up to the system builder.
In short, I build the hardware, update BIOS and if it applies, firmware in HDD, SSD, ODD and GPU, wipe the drive then install the Windows OS. After that it's off to find the usual drivers and apps. In this order. Motherboard, audio, video, LAN, apps and such. I never have the step by step since I don't own your gear. Only guides.
This will be my second motherboard and CPU that I have gone through. This is the Intel I76700K 8M 4.2G R with the motherboard as a MB ASUS Z170-A RTL.
Originally I started with a motherboard, I believe it was ASUS and an AMD CPU. The back ports went out so I bought a crossblade ranger motherboard and installed it myself. I booted it up and immediately noticed FPS drops across the board. I have a GTX Nvidia 660. Overwatch was previously running at higher than 60 fps and the upgrade brought it down to 40fps on low whereas I was running on high. I was distraught. So I save up again for my current CPU and Motherboard, even investing in a new heat sink.
I have installed both and have come to the similar conclusion. My fps is even lower at 10fps. I went through several guides on how to upgrade mobo without reinstalling windows, including deleting drivers and such. I had reinstalled windows on my second mobo (the ASUS Crossblade) to no success, still giving me less than optimal fps. I don't think a fresh install of windows is going to solve my problems, but as I speak I am working on reinstalling it. I deleted a ton of hidden drivers, even using the optimize feature on NVDIA experience. NVIDIA claims that I should be able to play on ultra with my rig, but I can't even get on medium with 10 fps. I'm so lost, I feel like I've failed at every step. Any help would be appreciated. I don't know what to do at this point.