IIS 10 / PHP. Upgraded server from Windows Server 2012 (IIS8.5) to 2019 (IIS10). Site degrades after X days on being online [closed]

Question

So I have a Production API site that gets alot of traffic and has performed fine for the past 5+ years.

We recently upgraded our Servers from Windows Server 2012 (IIS8.5) to Windows Server 2019 (IIS10). Same code, IIS setup, PHP, etc.

After 3 to 7 days when cutting over to the new Windows Server 2019, the site immediately starts to degrade and is brought to its knees. All requests timeout and in IIS Requests (worker processes), requests start queueing up and keep building up in Time Elapsed and never are completed. During this time CPU, memory, File I/O are all normal, but the server feels very sluggish when navigating through Windows.

Switching back to the Original Windows Server 2012, everything goes back to performing normally.

Steps we have tried:

1. Build a Windows Server 2019 from scratch and setup IIS sites, push code, etc.
2. Clone the working Windows Server 2012 and upgrade it to 2019.

Both of these result in the same issue.

Observations:

1. We have 3 other APP servers that we have successfully upgraded with no issues (Same code, PHP, same IIS site setup, etc.) - However the API site is hit more on the API server in question. As I said before, this same setup works fine on Windows Server 2012.