Shaders on iPhone 4

Guys, I’m writing a game on oF for iPhone. And I have a question.

I wanna draw a background with a shader.
For testing I just used a simple gradient fragment shader.
And I have like 6fps in the end which is turbo slow. When like nothing is going on in the game.
I realize that iPhone 4 is old but i checked other games and apps (for example shadertoy app) on the same phone and - they run quite fast with much more complicated shaders.
I’m wondering WHY in my case everything is so slow?
I’m basically drawing a shader to an ofFbo and then drawing while draw() method.