200x200 is nothing.

Your problem is you are creating UI widgets for all the cells upfront. This is bad.

You should have a data structure that represents your sheet as pure data. e.g. simple 2D array of Cell struct where each struct is basically a string and a double, or whatever it is you feel is the right way to represent the data behind the cell.

I did this kind of thing in Javascript and I can assure you that initializing a 2D array of simple objects is really fast. I can do 200x10000 array of objects in javascript and it would initialize in like 100ms or so.

Then when you render, you have a fixed number of cell widgets that you recycle all the time to only render the cells that are currently in view.