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Practices for Slippage-safe Strategies

In reference to this video, at around 21:10, there's a slide on characteristics of strategies that face relatively lower slippage risk. Could anyone please explain why:

  1. the expected paper PnL should be less than twice of spread, and
  2. the order sizes should be less than 25% of 1-minute participation rate?

Even though the rest of the points mentioned in the slide are clear, I can't understand the logic behind the above 2 points.