Pixel shaders are intensive even on desktop gpus. There are some differences you should be aware of:
Mobile gpus use a fixed structure where a shader processing unit can only perform one task. On desktop gpus, they can perform multiple tasks. This means that mobile devices have even weaker pixel shader performance than you'd expect comparing the GFLOPS, and 3dmark, as I understand it, isn't a good measure for just comparing pixel shader performance alone.
Tegra 3 has 12 shader units, 8 are for pixels, 4 are for vertex (12 GFLOPS, 1.04 gigapixels per sec pixel fill rate)
Tegra 4 essentially has 72, 48 are pixel shaders and 24 are vertex units (43.2 GFLOPS, 2.68 gigapixels per sec pixel fill rate)
iPhone 5s - couldn't find number of shader units, but performs at 76.8 GFLOPS
My 9800gt has 112 multipurpose units (504 GFLOPS, 9.6 gigapixels per sec pixel fill rate)
This means that even top level mobile gpus still don't even come close to my 9800gt, which is basically a rebranded 8800gt, that was released in 2007. A tegra 4 has less than 9% of the GFLOPS, compounded by the fact that 1/3 of its shader units can't be used for pixel shaders. They might seem powerful lately, but mobile gpus are really still quite weak.
As I recall, my 9800gt can't even handle somewhere around 50 small sprites with the tint effect applied to them at 60 fps. Pixel shaders are crazy intensive, so it's no surprise that mobile can't handle them all that well.