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Nevertheless, do you know how NDSolve solves this problem? The process is quite involved of course, but the part relevant to your question is, NDSolve needs to transform the equation to some kind of standard form, which is controled by EquationSimplification.

Currently there exists 3 possible option values for EquationSimplification: Solve, MassMatrix and Residual. By default NDSolve will try Solve first. What does Solve do? It'll transform the equation to the following form:

Nevertheless, do you know how NDSolve solves this problem? The process is quite involved of course, but the part relevant to your question is, NDSolve needs to transform the equation to some kind of standard form, which is controlled by EquationSimplification.

Currently there exist 3 possible option values for EquationSimplification: Solve, MassMatrix and Residual. By default NDSolve will try Solve first. What does Solve do? It'll transform the equation to the following form:

Finally, notice there exist many issues related to EquationSimplification -> Residual and I've avoid talking about most of them in order to keep this answer clean. If you want to know more about the topic, search in this site.

Finally, notice there exist many issues related to EquationSimplification -> Residual and I've avoided talking about most of them in order to keep this answer clean. If you want to know more about the topic, search in this site.

So, when setting Method -> {“EquationSimplification” -> “Residual”} / SolveDelayed -> True, you're turning to a cheaper transforming process for your equations.

So, when setting Method -> {"EquationSimplification" -> "Residual"} / SolveDelayed -> True, you're turning to a cheaper transforming process for your equations.