Same problem for us. Husband and I started a new Meadowlands farm with online co-op on Switch last night. We were both experiencing issues as soon as the game started so not a single hat or even pet on the farm. Husband experienced a hard crash on Spring day 5. We were both in the mines at the time.
He was host and his lag seemed less significant and not as often as mine. I was freezing up for 5+ seconds at times. He was playing with the Switch docked and I was playing handheld.
Our original game we started prior to the Ginger Island update eventually experienced lag after years in game, but only when running around. It was annoying, but nowhere close to game breaking. This new game/patch lag is happening no matter what’s going on. Even in the middle of fishing which is pretty significant gameplay breaking when I lag for 5+ seconds.