Investigate String Measurement in FX (cache results, call less, ...)

It is no secret that the cost of string measuring can have a huge effect on performance. String measuring operations are called often in FX to determine the preferred size of controls and layout happens often in FX as application code changes the contents of controls.

It's easy to see the same strings being measured over and over again. We could fix the callers to cache/call less or cache previous results way down deep inside of Prism.