Legend of Zelda: Breath of the Wild has had some frustrating slowdowns in the game as frame rates can drop to about 20FPS in some places. However, a new 1.1.1 patch seems to have resolved–or at least improved–those issues, although you couldn't tell it from the patch notes.
What exactly was fixed in the patch is left to your eyes because of Nintendo's frustrating practice of giving players the vaguest of details when it comes to changes. These patch notes give you all of one sentence: "Adjustments have been made to make for a more pleasant gaming experience." The patch is apparently out for the Wii U version as well.
Users on reddit seemed to confirm that frame rates were much better after they patched their game. "Tested this out a little while ago in battle, Kakariko Village, Hateno and several forest areas, both in docked and undocked modes on the Switch," user ElMechacontext said. "The difference is tremendous in Hateno village, and decent in forests. The sudden drops that are present in Kakariko are gone in both modes."
Whether the issues were optimization problems or just outright bugs will probably never be known, given Nintendo's vaguery, but the good news is that the game does indeed seem to offer "a more pleasant gaming experience." So at least the patch notes are correct.
YouTuber ElAnalistaDeBits did a comparison of 1.1.0 and 1.1.1 just for some visual confirmation.