Substance Painter relies a lot on the GPU to perform most of its computations. This is why it is important to have a GPU that follows the Technical Requirements.
The software works by transferring textures into the GPU memory (VRAM) in order to do the computations (like blending operations to create the final textures). However if the VRAM is starting to get full, the unused textures will be transferred back to the RAM of the computer to free some space. Substance Painter writes and reads GBs of data when working. This means that both the capacity of the VRAM (amount) and the bandwidth speed when doing transferts are important. You can use tools such as MSI AfterBurner to monitor this behavior.
The Nvidia GTX 970 is known to have a problematic design regarding its GPU memory that affects Substance Painter. The last 500MB of the whole 4GB work at a slower pace than the remaining 3.5GB. If Substance Painter works on those last 500MB, the performance can be reduced by 10 times factor (from what we measured). For more technical details, see : https://www.pcper.com/news/Graphics-Cards/NVIDIA-Responds-GTX-970-35GB-Memory-Issue