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