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I don't think season_length is something you should change unless your series has multiple seasonalities in which case you should probably use MSTL. If your series has only one seasonality then that's what you pass to season_length. Also, for longer seasonalities it always takes longer to fit specially with ARIMA. I don't see anything weird about that.

Well yes, we only want to use yearly seasonality. But is we give seasonality = 52, it`s up to 4x slower than when seasonality =1 . that`s what`s weiiiiird