In addition to Akash's tips to optimize LR5, I'd suggest to upgrade to LR6. Its just about 80 bucks and you get some nice new features.
The article does say:
Lightroom CC/Lightroom 6 can use a compatible graphics processor (also called a graphics card, video card, or GPU) to speed up adjusting your images in the Develop module.
so finding the tab in 5.7.1 was never likely to happen.
Pretty sure you misread something. GPU acceleration is new to 6.0.
The link you referred to says:
"...regarding GPU support in Lr CC (2015).
For example, on my test system with a 4K display, adjusting the White Balance and Exposure sliders in Lightroom 5.7 (without GPU support) is about 5 frames/second -- manageable, but choppy and hard to control. The same sliders in Lightroom 6.0 now run smoothly at 60 FPS.
Camera Raw Engineer"
How do you understand that to mean that there is a secret way to enable GPU processing in 5.7.1 but the head of the LR design team doesn't know about it?