How "Ground Effect" affect an aircraft’s performance?

The performance of an airplane is influenced by numerous external conditions. "Ground effect" refers to an illustration of one element. Ground effect is the effect of the ground or water on the airflow around the airplane. Whenever an airplane takes off, lands, or just flies very close to a surface, the stream of airflow is disturbed. Therefore, airplane performance is affected too. There are three major airflows that are affected: upwash, downwash, wingtip vortices. 

Will ground effect increase or decrease airplane performance? A reduction of an airplane induced drag will significantly increase airplane performance. I would like to explain why each component out of the three I mentioned is affected. First, let's look at the upwash and downwash. The more airflow wash there is, the bigger the force needed to push the air mass; therefore, the more drag is created. In the picture below we can see the size of the downwash inside and outside ground effect. 

We can see that inside ground effect (when the airplane is closer to the ground), there is less of a downwash. Therefore, less force is needed which equals less induced drag. (Less drag = better performance).

Looking at the effect of the ground on wingtip vortices we can conclude the same principle. A reduction of induced drag occurs. As a wing produces lift, it leaves behind it a circular pattern of whirling air around its wingtips, known as wingtip vortices. The ground prevents the high-pressure air from flowing to the low-pressure side. Induced drag is reduced because of the barrier of the ground to fully form wingtip vortices. (It equals to higher performance). 

References:

Federal Aviation Authority. (n.d.). Chapter 5 - Aerodynamics of Flight. FAA. Retrieved from

https://www.faa.gov/sites/faa.gov/files/regulations_policies/handbooks_manuals/aviation/phak/07_phak_ch5.pdf

This is how winglets work. (n.d.). Online Flight Training Courses and CFI Tools | Boldmethod. https://www.boldmethod.com/learn-to-fly/aerodynamics/how-winglets-reduce-drag-and-how-wingtip-vortices-form/