The way they want this tool to behave is adding a copy of all fiends but each new field name will start with an "X" and then the field name. I'm starting to think that the way arcmap adds a field is making a copy of the data somewhere, then adds the fields and then it puts the data back. That would explain why the time is not very significant for small datasets (the problem is that the real ones are big).
I am thinking making a copy of the layer, lets say, a temp layer, it would take about one or two minutes for big datasets. Then add the fields which will take a seconda adding all, and then move the data back to the original layer, delete de temp layer and the whole process would be around 3-4 minutes which is something more acceptable than 90 minutes.
If you know another way, please let me know.