Calculate/expose the time required to load each widget.

### Description

I'm new to Glance and I'm trying to optimize for a faster loading dashboard. Right now I've tried to upping the cache of many of my widgets to either one hour or even one day but there are still some of them I need refreshing sooner. Presently, it seems that basically any time I open Glance it needs to refresh and I'm trying to understand what forces it to and if there are any outliers when it comes to loading time.

I was thinking, then, that by having the time required for each widget to fully refresh in the background would allow us to benchmark and see if there's any one that is problematic, requiring too much time and dragging it all down to a slower refresh rate.

Start by tracing how widgets refresh in the background and how their cache durations affect loading when the dashboard opens. Expose the time each widget takes to fully refresh so users can identify slow outliers and understand what is delaying the dashboard refresh.

Written by the indexing model from the issue text.