Hi @sns,
Thank you for posting in the community.
In your question, is the page slow to open or is the page slow to open when you activate a site feature? Do you have this problem with all sites?
According to my knowledge, when you activate a site feature it is very slow, 30-40 seconds is normal, for example when you activate the publishing infrastructure feature.
It's hard to be sure that the slow page opening is due to a certain application consuming a lot of memory, because for example your site has a lot of features activated which can also cause a slow page opening.
For your first question, the memory consumed by the application is approximately equal to the size of its database. You can use this PowerShell command to see the size of all the application's databases.
Get-SPDatabase | Sort-Object disksizerequired -desc | Format-Table Name, @{Label ="Size in MB"; Expression = {$_.disksizerequired/1024/1024}}
For your second and third questions, I'm sorry that we can't see in the worker processes which application is causing high memory. Maybe you can find some three-party tools to view this information.
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