With (apply + (iota 1000)) and #:parallelism 1

serial: Result: 499999500000 in 55.959318191 seconds
4: Result: 499999500000 in 72.587197658 seconds, speedup of 0.7709254523732477
3: Result: 499999500000 in 61.389167076 seconds, speedup of 0.9115503737283513
2: Result: 499999500000 in 57.065367719 seconds, speedup of 0.9806178498060963
1: Result: 499999500000 in 54.25326041 seconds, speedup of 1.031446179789142

real    5m1.940s
user    9m53.504s
sys     0m7.056s

With (apply + (iota 1000)) and taskset -c 0

serial: Result: 499999500000 in 66.097115687 seconds
4: Result: 499999500000 in 112.084443247 seconds, speedup of 0.5897082036741002
3: Result: 499999500000 in 81.546630457 seconds, speedup of 0.8105437994995193
2: Result: 499999500000 in 68.884462606 seconds, speedup of 0.9595359125476111
1: Result: 499999500000 in 64.195950292 seconds, speedup of 1.0296150362499878

real    6m33.289s
user    6m30.468s
sys     0m1.472s

With (apply + (iota 1000)) and taskset -c 0-5

serial: Result: 499999500000 in 62.87166587 seconds
4: Result: 499999500000 in 48.105714106 seconds, speedup of 1.3069479798483714
3: Result: 499999500000 in 36.694999964 seconds, speedup of 1.713357839805992
2: Result: 499999500000 in 34.209348003 seconds, speedup of 1.8378504572635073
1: Result: 499999500000 in 34.974813469 seconds, speedup of 1.797626910168554

real    3m37.367s
user    11m2.400s
sys     0m30.556s