You are right, I just read the sentence on array access and did not read the sentence on vector access.

According to the GLSL spec the only way to access members of a matrix is using array subscripting syntax:


5.6 Matrix Components

The components of a matrix can be accessed using array...

The code looks good.

Do you have multiple windows? wglSwapIntervalEXT() is setting the swap interval for the window/DC associated with the current context.

NVIDIA drivers by default synchronize swaps to vblank, ATI driver do not. The 20ms look like vblank synchronization is enabled.
You can change the synchronization on swap using the...

Without seeing the source code is really hard to guess what's going wrong. Did you add calls to glMemoryBarrier() at the appropriate places?

The code looks fine.
I assume you are using NVIDIA hardware. There had been a bug with compute shaders and textures which is now fixed. Have you tried the latest drivers? Release 310.90 has the fix.