multsum | Summarization system taking multiple sentence | Natural Language Processing library

If I use discard instead of a variable name, does it have performance benefit? eg. by reducing assignment operations.

In your particular case it is unlikely that there would be a benefit in performance. The tuple that is returned is assigned to temporary storage; you've just not given a name to one part of that storage.

Now, if you had an expression that had discards that were entire values, not fragments of a tuple, then the compiler and the jitter can be smart about not allocating any storage on the short-term pool for the result, or re-using existing storage that was already allocated. Note that by "short-term pool" I effectively mean "activation record on the stack" or "registers". This could, in theory, lead to better register allocation or smaller frames (and therefore better locality of reference) and that in turn could save you entire nanoseconds.

Nano-optimizations are generally not worth it; there is almost always a better bang-for-buck performance problem to attack. But if you think it might be relevant for your scenario, measure it and see. That is the only way to know if there is a relevant performance difference. Get out a nano-scale stopwatch, run the code both ways, and see which one is faster.

The benefit you should be attempting to accrue by using discards is the "make my program easier to understand" benefit. Programmers are expensive; optimize for making your code easy for future programmers to read, understand and modify.

Is there any way to write the MultSum smart enough so it doesn't calculate the discards!?

Yes. Write your program in Haskell. Haskell will avoid performing calculations whose results are never used. C# is not such a language.