Difference Between Emulsion Speeds

In general, what's the difference between emulsions of different speeds? Are they different chemically, or just in the proportions of certain components? For instance, is TMY just TMX with 4x the concentration of something, or is it chemically different? Can one take any emulsion and make it more or less sensitive by changing certain proportions, or does it require a completely different formulation?

Each emulsion has several types of grains inside, and those react -more or less- depending on light intensity. Indeed those diferently sized grains can be ones inside others, like Russian matrioskas... I'd say every film design becomes a new world at least in part, so I'd say no: things are too complicated to be close to what you said.

Juan Valdenebro said:

Each emulsion has several types of grains inside, and those react -more or less- depending on light intensity. Indeed those diferently sized grains can be ones inside others, like Russian matrioskas... I'd say every film design becomes a new world at least in part, so I'd say no: things are too complicated to be close to what you said.

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That kind of confirms part of my suspicion - that it's completely different from a material point of view. I've been working through a couple of concepts related to developer action, solvability, speed, fog, etc., and it just doesn't make sense if there aren't substantial chemical differences.