Thanks for the quick response! I looked at several release notes too, to try to find this issue. Unfortunately, our users who have had this issue waited months to let us know about it.
I have personally had it on my computer for many months. I have used my computer for testing in Acrobat Pro and 2 different versions of Nitro Pro so I had wrongly attributed my delays to Acrobat hampering things. I tried deleting a font dat file I learned of in a Nitro article in \AppData\Roaming\Nitro\Pro\12. That did not seem to fix this.
As an example, earlier today I already had Nitro Pro open. Someone sent me a small PDF to look at and it took 10 seconds to open in Nitro, even though Nitro already had another PDF open. This makes it hard to isolate in testing. I did some testing yesterday and used a stopwatch to time opening in several different scenarios (over VPN, PDF in Outlook, PDF on Windows Desktop, PDF in OneDrive, etc.) and it opened quick enough (~3-5 seconds) with average size PDFs.