Once you figure out a segmentation workflow, it is usually quite easy to automate it. See for example how the automatic lung segmentation module works in LungCTAnalyzer extension. Initially it took lots of practice and several minutes to segment the lungs, now it just takes about 10 clicks in the lungs and you get segmentation very quickly and robustly. You could similarly create automated heart segmentation module, for example by automating this region growing based workflow.
Of course, nowadays for automated segmentation most people would train a neural network. NvidiaAIAssistedAnnotation extension allows you to use any NVidia Clara compatible models in Slicer. You can find readily usable models for many common segmentation tasks. There are also specialized AI-based segmentation extensions, such as DeepInfer that comes with automatic prostate segmentation models, or this lung segmenter extension.