gistfile.lua

collectgarbage('stop')
local MAX = 1000000

print("50: for+..", clock.bench(function() for j = 1, MAX do local s = "" for i = 1, 50 do s = s .. i end end end)[1])
print("50: for+table.insert", clock.bench(function() for j = 1, MAX do local s = {} for i = 1, 50 do table.insert(s, i) end table.concat(s) end end)[1])
print("50: for+table", clock.bench(function() for j = 1, MAX do local s = {} for i = 1, 50 do s[i] = i end table.concat(s) end end)[1])
print("50: for+table+new", clock.bench(function() for j = 1, MAX do local s = table.new(50, 0) for i = 1, 50 do s[i] = i end table.concat(s) end end)[1])

print("100: for+..", clock.bench(function() for j = 1, MAX do local s = "" for i = 1, 100 do s = s .. i end end end)[1])
print("100: for+table.insert", clock.bench(function() for j = 1, MAX do local s = {} for i = 1, 100 do table.insert(s, i) end table.concat(s) end end)[1])
print("100: for+table", clock.bench(function() for j = 1, MAX do local s = {} for i = 1, 100 do s[i] = i end table.concat(s) end end)[1])
print("100: for+table.insert+new", clock.bench(function() for j = 1, MAX do local s = table.new(100, 0) for i = 1, 100 do table.insert(s, i) end table.concat(s) end end)[1])
print("100: for+table+new", clock.bench(function() for j = 1, MAX do local s = table.new(100, 0) for i = 1, 100 do s[i] = i end table.concat(s) end end)[1])

--[[
result:
50: for+..	5.137557
50: for+table.insert	7.283428
50: for+table	6.728287
50: for+table+new	4.62105

100: for+..	15.128278
100: for+table.insert	14.525568
100: for+table	12.227589
100: for+table.insert+new	13.138442
100: for+table+new	9.673224
]]

here are some larger strings:

local clock = require('clock')
local fun = require('fun')
local MAX = 100000

local function bm(fn)
    return clock.bench(function()
        for _ = 1, MAX do
            fn()
        end
    end)
end

local function run(n)
    local strs = fun.range(n):map(function(x) return ('a'):rep(300) end):totable()
    print('' .. n .. ': for+..', bm(function()
        local s = ""
        for _, str in pairs(strs) do
            s = s .. str .. str
        end
    end)[1])
    print('' .. n .. ': for+table.insert', bm(function()
        local s = {}
        for _, str in pairs(strs) do
            table.insert(s, str .. str)
        end
        table.concat(s)
    end)[1])
    print('' .. n .. ': for+table', bm(function()
        local s = {}
        for i, str in pairs(strs) do
            s[i] = str .. str
        end
        table.concat(s)
    end)[1])
    print('' .. n .. ': for+table+new', bm(function()
        local s = table.new(n, 0)
        for i, str in pairs(strs) do
            s[i] = str .. str
        end
        table.concat(s)
    end)[1])
end

run(50)
run(100)

50: for+..	10.69397
50: for+table.insert	1.271426
50: for+table	1.061971
50: for+table+new	1.018673
100: for+..	43.339186
100: for+table.insert	2.587557
100: for+table	2.150612
100: for+table+new	2.061154

with :rep(30)

50: for+..	1.185899
50: for+table.insert	0.422421
50: for+table	0.342766
50: for+table+new	0.296029
100: for+..	4.399439
100: for+table.insert	0.886914
100: for+table	0.649508
100: for+table+new	0.572523

local clock = require('clock')
local fun = require('fun')
local MAX = 100000

local function bm(name, n, fn)
    print('' .. n .. ': ' .. name, clock.bench(function()
        for _ = 1, MAX do
            fn()
        end
    end)[1])
end

local function run(n)
    local strs = fun.range(n):map(function(x) return ('a'):rep(300) end):totable()
    bm('for+..', n, function()
        local s = ""
        for _, str in ipairs(strs) do
            s = s .. str .. str
        end
    end)
    bm('for+table.insert', n, function()
        local s = {}
        for _, str in ipairs(strs) do
            table.insert(s, str .. str)
        end
        table.concat(s)
    end)
    bm('for+table', n, function()
        local s = {}
        for i, str in ipairs(strs) do
            s[i] = str .. str
        end
        table.concat(s)
    end)
    bm('for+table+new', n, function()
        local s = table.new(n, 0)
        for i, str in ipairs(strs) do
            s[i] = str .. str
        end
        table.concat(s)
    end)
end

run(50)
run(100)

300
50: for+..	10.411794
50: for+table.insert	0.893488
50: for+table	0.807473
50: for+table+new	0.630997
100: for+..	39.144871
100: for+table.insert	1.65558
100: for+table	1.395289
100: for+table+new	1.20343

30
50: for+..	1.049417
50: for+table.insert	0.275828
50: for+table	0.249313
50: for+table+new	0.179999
100: for+..	3.888137
100: for+table.insert	0.571232
100: for+table	0.470101
100: for+table+new	0.370496

True, on my desktop:

tarantool -l clock bench2.lua
50: for+..	32.340517
50: for+table.insert	0.54341700000001
50: for+table	0.417396
50: for+table+new	0.22173099999999

100: for+..	124.873568
100: for+table.insert	1.038552
100: for+table	0.72050400000001
100: for+table+new	0.47638400000002

Ok, it seems that for + .. faster only for short strings and when we concatenate less than hundred of them. In other cases table.concat is much faster.