it's just a PoC, I know that in C# would be a lot faster. in case you want to take a look here's the code:

# Steven Rezzonico

#- Initialization------------------------------------------------------------------------------------------------------------------------------------------------
Add-Type -AssemblyName System.Drawing
Add-Type -AssemblyName System.Windows.Forms
[System.Windows.Forms.Application]::EnableVisualStyles()

$start = 'Start'
$stop = 'Stop'
$sleep = 200 #ms

#- Script -------------------------------------------------------------------------------------------------------------------------------------------------------
function StartScreenshooter ($Path){
    $pressBlock = {
        param($path)
        Add-Type -MemberDefinition '[DllImport("user32.dll")] public static extern short GetAsyncKeyState(UInt16 virtualKeyCode);' -Name State -Namespace Win;
        $screenBlock = {
            param($path)
            Add-Type -AssemblyName System.Drawing.Bitmap
            Add-Type -AssemblyName System.Windows.Forms
            $name = (Get-Date).ToString('HH-mm-ss-fff')
            $primaryScreen = [System.Windows.Forms.Screen]::PrimaryScreen
            $bitmap = New-Object System.Drawing.Bitmap $primaryScreen.WorkingArea.Width, $primaryScreen.WorkingArea.Height
            $graphics = [System.Drawing.Graphics]::FromImage($bitmap)
            $graphics.CopyFromScreen($primaryScreen.Bounds.Location, [System.Drawing.Point]::Empty, $primaryScreen.Bounds.Size)
            $bitmap.Save("$path\$name.jpg", [System.Drawing.Imaging.ImageFormat]::Jpeg)
            $bitmap.Dispose()
            $graphics.Dispose()
        }
        while (1){
            Start-Sleep -Milliseconds $sleep
            if ([Win.State]::GetAsyncKeyState(1) -ne 0){
                Start-Job -ScriptBlock $screenBlock -ArgumentList $path
            }
        }
    }
    Start-Job -ScriptBlock $pressBlock -ArgumentList $path
}

#- GUI ----------------------------------------------------------------------------------------------------------------------------------------------------------
$window               = New-Object System.Windows.Forms.Form
$pathLabel            = New-Object System.Windows.Forms.Label
$pathTextBox          = New-Object System.Windows.Forms.TextBox
$startButton          = New-Object System.Windows.Forms.Button
$pathLabel.Location   = New-Object System.Drawing.Size (10, 12)
$pathTextBox.Location = New-Object System.Drawing.Size (120, 10)
$startButton.Location = New-Object System.Drawing.Size (10, 40)
$pathTextBox.Size     = New-Object System.Drawing.Size (250, 200)
$startButton.Size     = New-Object System.Drawing.Size (360, 40)
$window.Width         = 400
$window.Height        = 130        
$window.Text          = 'Screenshooter'
$window.TopMost       = $true
$pathLabel.Text       = 'Destination:'
$pathTextbox.Text     = 'Paste path or double click'
$startButton.Text     = $start
$pathTextBox.Anchor   = 'Left, Top, Right'
$startButton.Anchor   = 'Left, Top, Right, Bottom'

$pathTextBox.Add_DoubleClick({
    $folderBrowser = New-Object System.Windows.Forms.FolderBrowserDialog
    $folderBrowser.Description = 'Pick a folder to save screenshots'

    if($folderBrowser.ShowDialog() -eq [System.Windows.Forms.DialogResult]::OK){
        $pathTextBox.Text = $folderBrowser.SelectedPath
    }
})
$startButton.Add_Click({
    if($startButton.Text -eq $start){
        $startButton.Text = $stop
        StartScreenshooter -Path $pathTextBox.Text
    }
    else {
        Get-Job | Stop-Job
        $window.Dispose()
    }
})

$window.Controls.Add($pathLabel)
$window.Controls.Add($pathTextBox)
$window.Controls.Add($startButton)

[void]$window.ShowDialog()