How much will dirty ducted fan blades affect the efficiency of the motor.

How much is a factor of how nasty the fan is but yes anything on the airfoil will degrade efficiency and worse possibly throw the system out of balance and at the speeds they spin is undesirable

I have a Freewing Vulcan 4S EDF and this time of year when the crabgrass is growing rapidly < even-though the runway is mowed at 1"every 3 days, I have trouble getting enough speed to take off. Yesterday I removed the EDF unit and the Blades and housing were coated with a layer of grass stain and debris. I cleaned them thoroughly and will try again Saturday with a new 4S 4200 battery.
I think the failure to get airborne was as much a fault of the dirty EDF as it was the grass???
Thanks for your response.

I have 2 freewing EDF's and while most times Ifind the power adequate, there are EDF's out there that have way more power and sound a lot better as well.Wemotec fans sound amazing but Jet Fans are like a big block in a car. The smoother the intake air, the nicer it sounds. Not sure about power. I think dirty blades may throw the fan out of balance, which will affect both power and sound.