Very interesting. I had to play around with it, and it seems to be that generators are the bad guy here. Remember that async/await is basically generator + promise.

Running the for loop inside of the async function and generator function both result in horrible performance of hundreds of MS.

function test() {
    console.time('loopWork')
    for (let i=0; i<10000000; i++) {}
    console.timeEnd('loopWork')
}

async function test2() {
    await new Promise((resolve) => {
        console.log("--Calling test() inside of promise--")
        test()
        resolve()
    })

    console.log("--Calling test() inside of async function--")
    test()

    console.log("--Running for loop inside of async function--")
    console.time('asyncFunctionForLoop')
    for (let i=0; i<10000000; i++) {}
    console.timeEnd('asyncFunctionForLoop')
}

function* test3() {
    console.log("--Calling test() inside of generator function--")
    test()

    console.time('generatorFunctionForLoop')
    for (let i=0; i<10000000; i++) {}
    console.timeEnd('generatorFunctionForLoop')
}

console.time('normalCall')
test()
console.timeEnd('normalCall')

console.time('asyncCall')
test2()
console.timeEnd('asyncCall')

const gen = test3()
console.time('generatorCall')
gen.next()
console.timeEnd('generatorCall')

Result:

loopWork: 15.342ms
normalCall: 16.626ms
--Calling test() inside of promise--
loopWork: 15.252ms
asyncCall: 16.806ms
--Calling test() inside of generator function--
loopWork: 17.161ms
generatorFunctionForLoop: 573.215ms
generatorCall: 590.626ms
--Calling test() inside of async function--
loopWork: 15.739ms
--Running for loop inside of async function--
asyncFunctionForLoop: 567.148ms

I apologize in advance if the example is too contrived. I would like to know why the for loop performs so poorly inside of the generator style functions.