Your nested loop over a 2D array runs much slower than expected despite simple operations. What should you check first?

Check the loop order: if the inner loop isn't varying the last array index, you're striding through memory and missing the cache on nearly every access.

Diagnosis:

// SLOW - column-major access (cache misses)
for (i=0; i<N; i++)
    for (j=0; j<N; j++)
        // j changes in inner loop but is first index!
        arr[j][i] = ...;

// FAST - row-major access (cache-friendly)
for (j=0; j<N; j++)
    for (i=0; i<N; i++)
        // i changes in inner loop and is second index
        arr[j][i] = ...;

Quick check: Is your inner loop variable the last array index? If not, swap your loops.

Other factors: Working set size (does data fit in cache?), stride length, memory allocation patterns.

Go deeper:

• Row- and column-major order (Wikipedia) — how storage order determines which traversal is cache-friendly vs cache-thrashing.
• CPU cache (Wikipedia) — cache lines pull in 64 contiguous bytes, so striding across rows wastes most of each line.