PowerShell is slow, but writing to files the way he is doing is just a bad pattern. He uses Add-Content not just in loops, but in nested loops. The problem with that is that each time Add-Content is called the system opens the file, adds the content, flushes to disk, and then closes the file. That's a lot of overhead.

In his wine glass script he also uses the PowerShell patten of creating an array ($x = @()) and then appending to it (foreach ($i in $set) { $x += $value * $i }). The problem here is that PowerShell arrays are fixed in size. To append a value to an array, the system creates a new array, copies all the values over with the new one, and then disposes of the old array. It works fine up to about 100 items, but it gets noticeably slow after that. Since it's done with a loop it makes it somewhere between O(n log n) and O(n^2). It's better to just output all the values as a single array ($x = $set | ForEach { $value * $_ }) or to create an ArrayList or List<String> or some C# collection that supports an O(1) append.

He also assigns a lot of variables in his loops and then uses them only once to format a string. He could eliminate those variables and just embed the expressions in the strings.

I got a ~25% performance improvement (210 ms to under 140 ms) with just the script embedded in the article when I switched to StringBuilder instead of Add-Content, and that script doesn't have the poor array pattern. StreamWriter would work, too, with less memory pressure than a StringBuilder. I suspect that with better code you could easily get this down below 3 seconds.