prepare uses measure text, if it is in a for loop, it won't be fast. This library is meant to do prepare once and then layout many times. layout calls should be sub-1 ms.
i think the intended purpose is that your text is maybe large but static and your layout just changes quickly. this is not the case for figuring out the height of 100k rows of different texts in a table, for example.
I am skeptical getting row height of many items only once is the intended behavior though. It is probably the intended behavior to get row height of many items and enables you to resizing width many time later (which is pretty useful on desktop).