#I have one project where I am importing a csv file and then I need to change the date format in two columns.

#I have completed few following loops where I am changing that format. So far for now, the fastest loop is the first one with ArrayList.

#Yes It's fast if you are working with hundreds or thousands records, but luckily I am working with ~200k records which takes around 2 minutes.

#So I just wanted to ask you if you know some another possibility how to do it.

$path = "$([Environment]::GetFolderPath('Desktop'))\SomeCSVFile.csv"

$RawCsv = Import-Csv -Path $path -Header ID, stime, etime, Process, User, Resource, FR

$Csv = $RawCsv | Select-Object -First 50000

$Stopwatch = [System.Diagnostics.Stopwatch]::StartNew()

########################################################################

$Stopwatch.Restart()
$arr = [System.Collections.ArrayList]::new()
foreach ($row in $csv) {
    $object = [PSCustomObject]@{
        ID        = $row.ID
        StartTime = (Get-Date $row.stime -Format 'yyyy-MM-dd HH:mm:ss')
        EndTime   = (Get-Date $row.etime -Format 'yyyy-MM-dd HH:mm:ss')
        Process   = $row.Process
        User      = $row.User
        Resource  = $row.Resource
    }
    [void]$arr.Add($object)
}
Write-Host "Array completed in: $($Stopwatch.Elapsed.TotalSeconds)"

########################################################################

$Stopwatch.Restart()
$out = foreach ($row in $csv) {
    [PSCustomObject]@{
        ID        = $row.ID
        StartTime = (Get-Date $row.stime -Format 'yyyy-MM-dd HH:mm:ss')
        EndTime   = (Get-Date $row.etime -Format 'yyyy-MM-dd HH:mm:ss')
        Process   = $row.Process
        User      = $row.User
        Resource  = $row.Resource
    }
}
Write-Host "Object completed in: $($Stopwatch.Elapsed.TotalSeconds)"

########################################################################

$Stopwatch.Restart()
foreach ($row in $csv) {
    $row.stime = (Get-Date $row.stime -Format 'yyyy-MM-dd HH:mm:ss')
    $row.etime = (Get-Date $row.etime -Format 'yyyy-MM-dd HH:mm:ss')
}
Write-Host "Update completed in: $($Stopwatch.Elapsed.TotalSeconds)"

########################################################################

$Stopwatch.Restart()
$csv.ForEach({ $_.stime = (Get-Date $_.stime -Format 'yyyy-MM-dd HH:mm:ss'); $_.etime = (Get-Date $_.etime -Format 'yyyy-MM-dd HH:mm:ss') })
Write-Host "DotForeach completed in: $($Stopwatch.Elapsed.TotalSeconds)"

########################################################################

$Stopwatch.Stop()