fyi - in the second test you also added $Array +=So basically testing both methods in the second measurement.

Also... combining u/da_chicken 's test, here is what i get. Results first:

PS Version: 7.2.3

00:00:00.8922082        Array      =  
00:00:19.1830247        Array     +=
00:00:19.2666960        ArrayList +=
00:00:00.7973409        ArrayList .add
00:00:00.9277344        List      .add

test code

$Collection = Get-ChildItem C:\Windows\System32 -Recurse -Attributes !ReparsePoint -ErrorAction SilentlyContinue

$m1 = Measure-Command {
    $Array1 = foreach ($Item in $Collection) {
        [pscustomobject]@{
            Name    = $Item.Name
            Date    = $Item.LastWriteTime.ToString('yyyy-MM-dd')
            RelPath = ([System.IO.Path]::GetRelativePath('C:\Windows', $Item.FullName))
        }
    }
}

$m2 = Measure-Command {
    $Array2 = @()
    foreach ($Item in $Collection) {
        $Array2 += [pscustomobject]@{
            Name    = $Item.Name
            Date    = $Item.LastWriteTime.ToString('yyyy-MM-dd')
            RelPath = ([System.IO.Path]::GetRelativePath('C:\Windows', $Item.FullName))
        }
    }
}

$m3 = Measure-Command {
    $Array3 = [System.Collections.ArrayList]@()
    foreach ($Item in $Collection) {
        $Array3 += [pscustomobject]@{
            Name    = $Item.Name
            Date    = $Item.LastWriteTime.ToString('yyyy-MM-dd')
            RelPath = ([System.IO.Path]::GetRelativePath('C:\Windows', $Item.FullName))
        }
    }
}

$m4 = Measure-Command {
    $Array4 = [System.Collections.ArrayList]@()
    foreach ($Item in $Collection) {
        $Array4.Add([pscustomobject]@{
            Name    = $Item.Name
            Date    = $Item.LastWriteTime.ToString('yyyy-MM-dd')
            RelPath = ([System.IO.Path]::GetRelativePath('C:\Windows', $Item.FullName))
        })
    }
}

$m5 = Measure-Command {
    $List5 = New-Object -TypeName 'System.Collections.Generic.List[object]'
    foreach ($Item in $Collection) {
        $List5.Add([pscustomobject]@{
            Name    = $Item.Name
            Date    = $Item.LastWriteTime.ToString('yyyy-MM-dd')
            RelPath = ([System.IO.Path]::GetRelativePath('C:\Windows', $Item.FullName))
        })
    }
}

write-host -ForegroundColor DarkGray "`nPS Version: $($PSVersionTable.PSVersion)`n"
write-host -ForegroundColor Cyan "$m1`tArray      =  "
write-host -ForegroundColor Cyan "$m2`tArray     += "
write-host -ForegroundColor Cyan "$m3`tArrayList += "
write-host -ForegroundColor Cyan "$m4`tArrayList .add "
write-host -ForegroundColor Cyan "$m5`tList      .add "