What units are those numbers in? I'm guessing they're frames per second?
To get some more intuitive numbers, you can calculate milliseconds per frame with
ms = 1000/fps.
Which gives: 3.85ms per frame, increased up to 7.69ms per frame, or an increase of 3.85 (a doubling in cost).
If you look at two different shaders:
vec4 colour = texture2D( F_color_texture, f_TexCoord );
final = colour;
vec4 colour = texture2D( F_color_texture, f_TexCoord );
vec3 normal = texture2D( F_bumpmap_texture, f_TexCoord );
final = colour * normal;
Then it's obvious that the second one does twice as much work as the first one... so you should expect this pixel shader to be double the cost of the first one.
If that's hypothesis is backed up by actual measurements, then it means that the pixel shader is the bottleneck in your program.
What kind of GPU do you have? ~3ms to fill the screen with a basic shader (one texture fetch) is quite slow.
What kind of objects are you drawing? How many vertices/triangles are being drawn? How many pixels are covered? What format are the textures? Do the textures have mipmaps, and do you have mipmapping sampling states enabled?