Do motion graphics on a landing page improve conversions or just slow everything down?

We're designing our marketing website, and our designer wants to incorporate animated illustrations, animated illustrations, motion graphics and subtle interactions throughout the experience. The mockups look fantastic but our developer is convinced they'll hurt page speed, especially on mobile and ultimately reduce conversions. Both sides keep pointing to studies that support their argument so I'm struggling to figure out which approach is actually right. Has anyone measured the impact of thoughtful motion design on landing page performance?