To support Shawn's opinion, yes, the amount of stress a flex animation would add to the simulation is not worth it. Either an animation with bones (which would be considered really cheap) or using $treesway would be better options.
Doing with bones is good because the effect can be mimicked very closely, and it's by far the cheapest. The advantage with $treesway is that it can look very realistic since it'll be affected by the env_wind entity, though it is probably more 'expensive' than just an animation. However, I think that the $treesway option takes advantage of the GPU, which is also good.