Hi,
Besides asset file size and network speed, the shader complication stage affects the loading much. This is because in WebGL, shaders are compiled rather slowly. So the less shaders your application uses, the faster is loading. Also, shaders are cached by WebGL, so if your shader is used by multiple objects it is compiled only once.
Therefore for fast loading, it is important to use as few unique shaders as possible. One of possible way to achieve this is to enable the glTF compatibility flag, which a) simplifies the shaders a lot so that they are compiled and work faster and b) standardizes them to enable for more effective caching.
The obvious drawback of using glTF-compatible shaders is that they may be too simplistic for implementing desired graphics effects. So as it is always in real-time graphics, you need to balance between quality and performance.