SwiftUI Is Convenient, but Slow
Other commenters have pointed to SwiftUI being declarative as a big part of this problem, but I take a more nuanced view: the problem is actually that SwiftUI is a complexity-hiding abstraction. Specifically it hides the complexity of when UI is refreshed and how it is refreshed.
This results in a situation where engineers who don’t have a clear understanding of how it works resorting to essentially guesswork to try and fix performance issues. Now, there are certainly folks who do have a deep understanding of how SwiftUI works but that group is likely small and the situation isn’t helped by middling documentation and SwiftUI being closed source. 
An analogue to this can be seen in something like SQL, which is a (somewhat) declarative specification for queries that is also complexity-hiding. SQL, wielded by a novice, can be pretty slow (just one ill-advised join would likely do it), but would still “work”. The difference with SQL is that there is a plethora of literature about how to write it in a way that results in fast queries and many of the consumers of SQL are open source so enterprising engineers can dig down and figure out what a particular query is doing and why something is slow. We’ve also invested a ton of resources in creating engineers who can write good SQL.
I’m interested to see how SwiftUI progresses in these regards.<p><i>Unrelated addendum:</i> The author should look at the drawingGroup API. This could significantly speed up the drawing of the month grids on refresh by caching the resulting rasterized view instead of repeatedly redrawing it.