A performance patch has been released for Stardew Valley. This patch fixes a number of performance issues, including a problem with the game's framerate when you have a particularly big farm. Hop on your Switch right now and the update should auto-download.
UPDATE - Here's the full patch notes
Improved performance in locations with lots of light sources (e.g. torches).
Improved performance on farms with lots of animals.
Tweaked new "H" health icon
Fixed multiplayer visual glitch when another player uses a tool and then starts moving in a new direction.
Fixed "fishing stance" visual glitches in multiplayer.