With Tensorflow and other libraries they are able to use GPU rather than CPU. Is this the same case with OpenCV? If this is the case, then additionally what graphics cards does it support? Does it support Nvidia, AMD and Intel-based graphics cards? If this is true then does it also support Raspberry Pi’s GPU as well?
depends on what modules of OpenCV you use.
DNN stuff can use OpenCL, CUDA, intel inference engine (library), …
for a long time Raspberry PI GPUs hadn’t even been documented enough to allow decent open source drivers. it’s also not that powerful. according to some sources, the VideoCore IV has ~24 Gflop/s of processing power. that’s less than low end desktop CPUs deliver these days. you should absolutely expect anything to run like molasses on a raspberry pi, even if you get the GPU to do the work.
the best you can hope for is that someone made an OpenCL driver for it, and then OpenCV might be able to use it, but you should expect this to be a struggle.