You should use the spoiler flag, especially for blue katas.

Nop.
I just run a benchmark to be sure, with smallNumber = 25 and bigNumber = Integer.MAX_SIZE.

Here are the results :

Benchmark Mode Cnt Score Error Units
MultiplesOf3And5Test.withForLoopAndBigNumber avgt 5 2.680 ± 0.055 s/op
MultiplesOf3And5Test.withForLoopAndSmallNumber avgt 5 28.495 ± 4.046 ns/op
MultiplesOf3And5Test.withParallelStreamAndBigNumber avgt 5 1.044 ± 0.184 s/op
MultiplesOf3And5Test.withParallelStreamAndSmallNumber avgt 5 11284.553 ± 1710.007 ns/op
MultiplesOf3And5Test.withStreamAndBigNumber avgt 5 3.013 ± 0.049 s/op
MultiplesOf3And5Test.withStreamAndSmallNumber avgt 5 94.385 ± 3.109 ns/op

(Yes I used warmup steps to remove JIT and CPU bias)

Here are my interpretations :

• The withForLoop and the withStream are very close with small numbers. The difference comes from the stream initialisation.
• The performance converge with BigNumber
• The parallel implem with default thread-pool stream is 2 time better than every other
• The parallel implem with custom thread-pool could be even better
• People should stop throwing performance asumptions without proper benchmark

Nop, the text says "below n" and not "equals or below n".
So for n=9, then 14 is the correct answer (3 + 5 + 6).

Also, if you really want inclusive IntStream, use IntStream.rangeClosed(start, end)