This diagram describes the physics behind what is called a cantilever beam. A cantilever is fixed on one end and free on the other. An endmill is a cantilever. The symbol δ is the deflection of a beam from a force (F) applied. The equation below the graph says the deflection (δ) is in proportion (∝) to the length of the beam cubed (l x l x l). Applying this to endmills, let's use a stick-out of one time the diameter (1X) as a baseline (1 x 1 x 1 =1). If we stick-out an endmill two times the diameter (2X) and apply the same amount of force as the 1X, the deflection does not double, it increases EIGHT TIMES (2 x 2 x 2 = 8). There are endmills on the market that have flute lengths of 7X the diameter. They will deflect at a rate 345 times that of a 1X (7 x 7 x 7 = 345)!
What does this mean for the endmill?
Greater deflection of an endmill means more time is required between the equal tooth impacts for stable cutting. That means for a longer stick-out, you have to either reduce the number of teeth (which lowers the feed rate) or lower the spindle speed. You could also reduce the force (F) to reduce the deflection by lowering the radial and/or axial depth of cut.
Here you can see the dramatic loss of performance from tool length. Sometimes you have no choice but to use longer endmills, but the further the way you get from spindle, the less performance potential you will have.