How can I figure out what's making my site slow?

Question

I'm maintaining a legacy ASP.NET MVC app (on .NET 4.8). One of my pages is unacceptably slow, often taking 30+ seconds to send a response to the browser. I've used MiniProfiler to try to find what's being slow. The results are below:

			duration (ms)	with children (ms)	from start (ms)
/Administer/ManageUsers			30530.16	37085.67	+0.00
	ManageUsers		414.11	5553.60	+3.93
		GetUsers	5118.44	5118.44	+418.00
		CreateModel	21.05	21.05	+5536.47
	Render: ManageUsers		236.36	1000.70	+5558.86
		Render partial: _ManageUsersFilter	726.86	726.86	+5677.21
		Render partial: _LoginPartial	13.40	13.40	+6495.95
		Render partial: _LoginPartial	11.18	11.18	+6520.76
		Render partial: _LoginPartial	10.52	10.52	+6532.08

I've thoroughly covered the controller in profiler calls (e.g., using (MiniProfiler.Current.Step("GetUsers"))). In stepping through the code with the debugger, it's clear that the controller accounts for about 20% of the total time. Where's the rest of it coming from?

According to this typical profiling result, the controller is taking about 5.5 seconds and rendering is taking about 1 second. What about the other 24 seconds, that are definitely occurring either during the controller return or after it.

Of course, there are many problems with the code I have the "pleasure" of maintaining, but what I'm trying to solve right now is what's taking those 24 extra seconds.