You can find the Movement Speeds settings in the BP_PlayerInventory then select the Class Defaults button to show the blueprint settings in the Details panel. In that panel you can use the search bar to search for Movement Speeds and you will see the settings like below.
Here you can set the movement speeds for each character state.
If you want to change the characters movement speed while holding a holdable item, then you can go to that items holdable blueprint like for example the BP_M4A4, then go to the class defaults and use the search bar to search for Movement Speeds to find the settings shown below.
Use Holdable Movement Speeds – If true the Movement Speeds values set in the holdable bp will be used instead of the Movement Speeds values set in the Player Inventory, this can be used to set different speeds for different held items.