Given the cost of abandonment and lost sales due to slow page rendering (there's a lot that's been written about that), even a 3% loss of conversions due to slow frontend code over 2 years is a lot of money. For a million-dollars-a-year business, that's upwards 6 months worth of a developer's time.

[0] 3% based on how many people will abandon a page that takes more than 1 second to render: https://blog.kissmetrics.com/loading-time/?wide=1