We are still on SharePoint Server 2010, using NTLM.
If a user is in the site collection administrators group, nearly every page across the site collection loads in 2 seconds or less. If a user is not in the group, it will take 7 seconds or more to load nearly every page.
We tested this on a user's computer that was complaining about the slowness. Tested using Developer Tools on IE 11, latest FireFox, latest Chrome - checking the network profiling to observe page loads from beginning to end. It will take 7 seconds or more to load any page. Using the same computer, browser, etc. we will make no changes but select the user name in the browser in SharePoint, then "Sign in as Different User". We will sign in as a user in the site collection administrators group. BAM, everything loads 2 seconds or less.
We suspected the AD groups of the user account were affecting load times - as SharePoint can verify permissions based on AD group membership. We created a new AD account and only added the domain users AD group and did not add the user to the site collection administrators group. We experienced slow load times. Then adding the new AD account to the site collection administrators group, load times are 2 seconds or less per page.
Any help appreciated!