Fps boost - That's how it works.
Fps boosting Texture Pack - That's how the insane FPS boost works
As you know are Minecraft's default textures/resources based on a 16x16 pixels resolution. That's why a 8x8 pack makes it able to get higher fps-rates than with other packs.
If you compare it with a 256x256 or 512x512 high resolution texture pack you actually easily should be able to get a lot higher FPS rates.
If there are 1000 Blocks your pc has to render in 512x512 resolution you may need good/expensive hardware because that's a total of 1572864000 (!) Pixels.
With 8x8 resolution your pc (only) has to render 384000 Pixels. That's a difference of 409600%.
You need good hardware, of course. But many people use fps-boosting texturepacks to easily reduce their pc-load to make using a shader/live-recording in high resolution possible.