Using LDS slower than recomputing?!

I was trying to solve this for a whole month now.
I am recomputing various times the same equation. The equation takes few float operations and even a sqrt.
I tried in various ways to compute the equation only once and store it in LDS and then read the result from there instead of recomputing it.

Surprisingly for me, i could not find a way to beat recomputing with LDS. Recomputing the same equation many times is the fastest option. I tried at least 4 different approaches/shaders. At least 4 main shaders and a lot of variations of each …