Which is correct between “and” or “but”

Question

Situation is like as follows:

The more A is reduced, the better the performance is.

The more B is reduced, the better the performance is.

The more C is increased, the better the performance is.

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The sentence is like this:

Since blah~blah~blah, both A and B have been significantly reduced, (and, but) C also has been increased.

1st thought: increased is a counterpart of reduced, so "but" is correct.

2nd thought: the former sentence gives better performance, the latter one also gives better performance, so "and" is correct.

I do not know which is correct.

Answer 1

I think your perplexity arises because you are working with a sentence fragment rather than an entire sentence.

Because A and B have been reduced and [because] C has been increased, the performance of D has been improved.

Use of but instead of and in the preceding sentence would be grammatical, but it would be inconsistent with the overall meaning of the complete sentence. The choice of the correct word depends on that overall meaning, not words or clauses considered in isolation from the rest of the sentence.

Answer 2

but indicates a contrast, while and and also indicates a similarity. In this case, A, B and C share two relations, a common cause "blah-blah-blah" and a common outcome "performance".

Now the "performance" doesn't matter at all in your example sentence. That's not the problem. The only problem you have is but also, and that's a minor problem here. It indicates both a similarity and a contrast. That is technically correct, because C is similar in being influenced in blah-blah, but different in the direction in which it is influenced.

This isn't a problem particular to English. You just need to choose what you are trying to communicate, the similarity or the difference. (if at all)

Since blah~blah~blah, both A and B have been significantly reduced, **and also ** C has been increased.

(Stressing similar cause)

Since blah~blah~blah, both A and B have been significantly reduced, but C has been increased.

(Stressing Different direction of change)

Since blah~blah~blah, both A and B have been significantly reduced and C has been increased.

(Not stressing either, just listing all three effects)

Note that but also does occur in the common pattern not only X but also Y. We here see that but indicates a contrast here, between (not) only X and X and also Y