$s=$PWD.Path
$o=Join-Path $s woffs
Get-ChildItem -LiteralPath $s -Recurse -File |
    ? { $_.Extension -match '^\.(ttf|otf)$' -and -not $_.FullName.StartsWith($o) } |
    ForEach-Object -Parallel {
        $rel=$_.DirectoryName.Substring($using:s.Length).TrimStart('\','/')
        $t=if($rel){Join-Path $using:o $rel}else{$using:o}
        [IO.Directory]::CreateDirectory($t)|Out-Null
        ftcli converter ft2wf -rtb -out $t -f woff2 $_.FullName
    } -ThrottleLimit 8