Minimal example to demonstrate performance difference between seq and array access

longOuter.nim

import times

template benchmark*(benchmarkName: string, code: untyped) =
  block:
    let timeStart = cpuTime()
    code
    echo benchmarkName
    echo cpuTime() - timeStart, "s"

var seqBase = @[@[1, 2], @[322146413, 397146891], @[2100521352, 391739920], @[
    330559900, 77684297], @[908332622, 428261606], @[1, 2], @[322146413,
        397146891], @[2100521352, 391739920], @[
    330559900, 77684297], @[908332622, 428261606]]
var arrayBase = @[[1, 2], [322146413, 397146891], [2100521352, 391739920], [
    330559900, 77684297], [908332622, 428261606], [1, 2], [322146413,
        397146891], [2100521352, 391739920], [
    330559900, 77684297], [908332622, 428261606]]

var
  someSeqOSeq = seqBase
  someSeqOArray = arrayBase

const
  nIter = 200
  nCopy = 100

for i in 0..nCopy:
  someSeqOSeq &= seqBase
  someSeqOArray &= arrayBase

benchmark "Seq of Seq":
  var count = 0
  for x in 0..nIter:
    for y in 0..nIter:
      for s in someSeqOSeq:
        let a = s[0]
        let b = s[1]
        if ((a + b) * 23) mod b == 0: count += 1

benchmark "Seq of Array":
  var count = 0
  for x in 0..nIter:
    for y in 0..nIter:
      for s in someSeqOArray:
        let a = s[0]
        let b = s[1]
        if ((a + b) * 23) mod b == 0: count += 1

main.nim

import times

template benchmark*(benchmarkName: string, code: untyped) =
  block:
    let timeStart = cpuTime()
    code
    echo benchmarkName
    echo cpuTime() - timeStart, "s"

let someSeqOSeq = @[@[1, 2], @[322146413, 397146891], @[2100521352, 391739920], @[
    330559900, 77684297], @[908332622, 428261606], @[1, 2], @[322146413,
        397146891], @[2100521352, 391739920], @[
    330559900, 77684297], @[908332622, 428261606]]
let someSeqOArray = @[[1, 2], [322146413, 397146891], [2100521352, 391739920], [
    330559900, 77684297], [908332622, 428261606], [1, 2], [322146413,
        397146891], [2100521352, 391739920], [
    330559900, 77684297], [908332622, 428261606]]

const nIter = 1000
benchmark "Seq of Seq":
  var count = 0
  for x in 0..nIter:
    for y in 0..nIter:
      for s in someSeqOSeq:
        let a = s[0]
        let b = s[1]
        if ((a + b) * 23) mod b == 0: count += 1

benchmark "Seq of Array":
  var count = 0
  for x in 0..nIter:
    for y in 0..nIter:
      for s in someSeqOArray:
        let a = s[0]
        let b = s[1]
        if ((a + b) * 23) mod b == 0: count += 1

using compiler built from commit 9eaf99f7e61d07f5502b3656a255ea944c47f984.
Result of main.nim from my machine:

• with -d:release:

Seq of Seq
0.45509076s
Seq of Array
0.1059315100000001s

• without:

Seq of Seq
4.405831273s
Seq of Array
0.4498233489999999s

Result of longOuter.nim from my machine:

• with -d:release:

Seq of Seq
1.941198598s
Seq of Array
0.3409378690000002s

• without:

Seq of Seq
18.001304946s
Seq of Array
1.463049079000001s