nothing to do with flight model, go try to understand first which external, and internal actions have an impact on wingflex and you will realize it is a function that requires several iterations to calculate the polygons for one specific point in time which again depends on its position and dynamics of a previous point in time. In order to respect material elasticity, weight distributions, dynamic forces etc etc. such models, when used to produce animations for static analysis in engineering as an example, are pre-calculated for given set of conditions and and then injected into a rendering system.
we are asking X-Plane to do this job real-time!
I am not saying it is impossible to achieve an animation that gives you an impression of a well simulated flex at low performance impact, but it has nothing to do with real wingflex. (Please, don't mention FF again, I cannot consider any of their add-ons comparable with what we are talking about here)
There is another question I ask myself though: I still wonder when I would be able to use that animation. The only moment we could really admire it is when we land, i.e in replay mode. On my side, replay is a black box, seems broken... custom animations not working well with almost any plane, IXEG even has the option to deactivate those in replay mode.
...now enough... I am going to fly.. without wingflex!