Heavy use of Unity API & UI Thread

Hi.

I need to add many Image in my Canvas. I'm using a loop that creates a gameobject and which adds the Image component every lap, beside that it makes freeze the UI thread for around 10 seconds. I tried to make those heavy computing inside another thread that should have worked but I receive an error message because I'm using Unity API outside the main thread. I also tried to uses coroutines and that works but it's very slow since it's waiting every loop lap to prevent the freeze.

Do you guys have any insights ?