When you're asked to generate a quick report from a bunch of data that's stored in CSV files you think, "Hey that's what spreadsheets are good at.", or if it's a lot of CSV files, "That's what PowerShell is good at." I only offer this as a caution, while it can do it, sometimes it still shouldn't be done. The PowerShell ISE get's a little dodgy when it's using more than 4 GB of data.
The script was still running at this point (1.5 hours in) and finished at 4700 MB used.