Stall in Image Store with Framebuffer Fetch

Hi

I used Samsung S9 (Mali-G72, OpenGL ES 3.2 r9p0).

I Implement a method to copy current pixel to a image.

I use Framebuffer Fetch to read pixel, then I use Image Store (not atomic) to a image.

And It will process a Dual Filter (Down-Up Sample) to do Bloom Texture in next frame.

But I used the method in OpenGL ES, It dropped 30fps

I see CPU is waiting for something. and GPU is slowdown.

Using the method in Vulkan is not problem.

It can see low usage on Mali L2 Cache Stall in Streamline.

So how to know what the problem ? (no problem in other vendor too)

Thanks