[Gameplay]- Items: Moblity Boots + Alacrity
[Gameplay] - Item
I took 
with 
. After I upgraded to 
but it didnt gave me + 20 mouvement speed. It gave me 16 mouvement speed instead. I don't know if it is only on or all other champions
2 Comments
This is working as intended, and works like this on all champions.
As you get higher movement speed, the movement speed you get has diminishing returns. I believe the first threshhold for the movement speed soft cap is at 420, and you already reached that with Mobility boots alone. Thresh's base movement speed is 335, and with the 105 movement speed on Mobility Boots, you should be at 440 without the soft cap, but instead you are at 435 in the first image.
Here is the Wiki article that explains this: http://leagueoflegends.wikia.com/wiki/Movement_speed