Applying an existing, trained model (like what you described) is computationally cheap. Training new models takes many, many orders of magnitude more time and memory space to complete.
Much cheaper than training for sure. But inference for image classification it is generally in the order of hundreds of milliseconds when using CPU.
That wont fly for real-time inference on video, for example. Which is very desirable for even "image" uses, because it allows for live preview in the camera app.
