Faster calculation histogram of a set of images
Python is among the slowest production-ready languages you can use.
As you haven't posted any code, I can only provide general suggestions. They are listed in order of practicality below:

Use a compiled version of python, such as pypy or cpython
Use existing software with your desired functionality. There's nothing wrong with finding free software online.
Use a more efficient (or perhaps even lossy) algorithm to skip computation
Use a faster language such as Rust, C++, C#, or Java
I have about 3 million images and need to calculate a histogram for each one. Right now I am using python but it is taking of lot of time. Is there any way to process the images in batches? I have NVIDIA 1080 Ti GPU cards, so maybe if there is a way to process on the GPU? 
I can't find any code or library to process the images in parallel. Any kind of help to boost up the speed is appreciated