It’s really as simple as looking at the goal differential:
In their last 15 games, the first period has been much worse than breaking even, as they have been outscored by nine and outshot by 17 in the first period. And keep in mind it hasn’t exactly been the toughest stretch of hockey in those 15 games – Buffalo three times, Edmonton twice, Ottawa, Vancouver, and the New York Rangers, with some difficult opponents mixed in there.
It’s worth noting that this particularly poor stretch is also seeing teams take advantage of a worse than usual Leafs defense due to injuries.