There can be a number of reasons why pages are slow, but one of the simplest reasons is that the data is not in a useable form in the database. Almost all of the problems I've fixed where the UI was completely unusable and the data would never load were due to poor SQL database queries, data types, and lack of indexes. Getting all 3 of these right can fix nearly every problem for "typical" webpages and reports. The difference they can make when combined is often 2 orders of magnitude, or 100x faster loading times.

Just because the client is using a custom dashboard isn't an excuse for slow loading, either. Slow loading times are frustrating and make people feel like the custom tool they are paying to use doesn't even work. The behind-the-scenes custom dashboards should be even faster than typical webpages for a couple reasons. First, they are usually being paid for, so give the client something that feels like it's worth it. And second, they often source the data from internal databases and infrastructure, which means there are fewer obstacles in the way of getting data ready for optimal delivery.

There's just no excuse. Stop letting things load slowly.