Why are elementwise additions much faster in separate loops than in a combined loop?

Question

Suppose a1, b1, c1, and d1 point to heap memory and my numerical code has the following core loop.

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Answer 1

It's not because of a different code, but because of caching: RAM is slower than the CPU registers and a cache memory is inside the CPU to avoid to write the RAM every time a variable is changing. But the cache is not big as the RAM is, hence, it maps only a fraction of it.

The first code modifies distant memory addresses alternating them at each loop, thus requiring continuously to invalidate the cache.

The second code don't alternate: it just flow on adjacent addresses twice. This makes all the job to be completed in the cache, invalidating it only after the second loop starts.