The speed could be automatically adjusted by the robo-vacuums by sensing the dirt intake. The sensing of dirt can be done by sensing static transfer from plate to plate along the path of air flow. A cover would need to make the plates accessible for occasional cleaning.

Do these vacuums dock themselves to recharge? Easily doable using differential infrared sensing. A low voltage interface would allow recharging without any danger to anyone touching the charging contacts.

Good idea, I am very surprised they didn't build this in to conserve power. You should pitch to the iRobot people that build the thing.