If it were only the sum() or only the fold() as on stackoverflow this would be true.
But benchmarking shows that combining map() and sum() is about 10% slower.
Also, using iter() instead of into_iter() will degrade performance by another 30-40%.

Using vectors with 100 million elements on my machine:
iter + map + sum = ~3.1-3.3s
into_iter +map + sum = ~2.5-2.6s
iter + fold = ~2.9-3.1s
into_iter + fold = ~2.3-2.4s