CommandBuffer.RequestAsyncReadback is stalling

While AsyncGPUReadback's "RequestToNativeArray" function can omit it's callback function and have it as null, the command buffer version can't. It throws an InvalidOperation exception because it does not take null as an argument.

By adding a callback that reads some data back from compute into CPU, the render thread is stalling.
On PC it is barely noticable, but the 0.04ms converts to 10ms on a low end device

If I could omit the callback, I could read data manually from the native array, even though it might be dirty, but without stalling

 

Actually, it seems that it is the dispatch is taking this long, and the readback command is waiting for it.
But the suggestion with "null" parameter still stands. The callback may not be necessary if RequestToNativeArray is used.