Diagnose data access latency

The code is pretty simple

kernel void naive(
    constant RunParams *param  [[ buffer(0) ]],
    const device float *A      [[ buffer(1) ]],     // [N, K]
    device float *output       [[ buffer(2) ]],
    uint2 gid                  [[ thread_position_in_grid ]]) {
    uint a_ptr = gid.x * param->K;
    for (uint i = 0; i < param->K; i++, a_ptr++) {
        val += A[b_ptr];
    }
    output[ptr] = val;
}
 

when uint a_ptr = gid.x * param->K, the code got 150 GFLops when uint a_ptr = gid.y * param->K, the code got 860 GFLops

param->K = 256; thread per group: [16, 16]

I'd like to understand why the performance is so different, and how can I profile/diagnose this to help with further optimization.