Upgrade from 3.80 to 4.40.3 - Site takes 15 mins to start & then hangs and again goes down

We are upgrading nopcommerce from 3.80 to 4.40.3. It works for our http://demo.xyz.com on live server. As soon as we change the mapping to our live URL https://www.xyz.com the site takes 15 mins to start and hangs and again goes down.

We tried everything in this post: https://www.nopcommerce.com/en/boards/topic/90943/extremely-slow-response-after-upgrade-from-440-to-4403

It doesn't resolve.

We tried the following scenarios for testing on live server:
Our upgraded 4.40 custom Code and our upgraded 4.40 DB mapped to our live URL http://demo.xyz.com - It works
Our upgraded 4.40 custom Code and our upgraded 4.40 DB mapped to our live URL https://www.xyz.com - It doesn't works
Fresh 4.40 code and fresh 4.40 DB mapped to our live URL https://www.xyz.com - It works
Fresh 4.40 code and our upgraded 4.40 DB mapped to our live URL https://www.xyz.com - It doesn't works
Our upgraded 4.40 custom Code and fresh 4.40 DB mapped to our live URL https://www.xyz.com - It doesn't works.

Please suggest a solution, if some has came across similar issue.

It could be a lack of memory.  Check your ASP .NET and IIS logs.

Checked logs.
It gives Max Pool Size Error

An exception was thrown attempting to execute the error handler.
Exception:
System.InvalidOperationException: Timeout expired.  The timeout period elapsed prior to obtaining a connection from the pool.  This may have occurred because all pooled connections were in use and max pool size was reached.

We have tried everything...it looks like nopcommerce is not able to fix the issues since they released 4.30

Any help appreciated!!!

Use SSMS (or similar tool), and query the Log table

select top 1 * from Log where ShortMessage like '%Timeout expired%'

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