I started a thread on cgshaders.org about this issue and heard that I'm not the only one that noticed this.
The lerp function (which translates to an ADD and a MAD in arbfp1.0 assembly) significantly lowers performance on my Geforce FX 5600.
With the fragment program disabled I get anywhere from 300-330 fps while with the fragment program enabled it lowers to 70 fps with a single lerp and it goes down to 25 with 4 lerps (using 5 textures).
Does anyone know the reason why?
For reference (shader source code is there too), here's a link to the other thread: http://www.cgshaders.org/forums/viewtopic.php?t=2244
Thanks for reading.