I tried a few options to iterate my directories and get a huge performance difference between the following commands:
Slow:
Get-ChildItem -Directory -Force -Recurse -Depth 3 -Include '$tf'
Fast:
Get-ChildItem -Directory -Force -Recurse -Depth 3 | Where-Object Name -eq '$tf'
Could someone explain me why the first statement is much slower than the second?
Get-ChildItem is a provider cmdlet - that means that a bulk of its actual work is offloaded to an underlying provider, likely the FileSystem provider in your case.
The provider itself doesn't actually support the -Include/-Exclude parameters, that's one of the few things that the cmdlet takes care off - and for the file system provider this is ultra heavy double-work, because the cmdlet needs to recurse down through the file system hierarchy to figure out whether it needs to apply an exclusion or an inclusion based on a parent directory name, you can see how this is implemented here.
So by using -Include against the file system provider, you're asking PowerShell to do an immense amount of double work.
