Tips on how to make reporting faster

In my experience, the reporting module never was really fast, but I'm now working on a project where a report takes roughly 5 minutes to create a 1 page PDF, which I think is unacceptably slow.

The report template contains a map, and I use the Generate Webmap for Report and Run Report activities in a workflow to create the PDF. The webmap contains ~20 layers, but only one of those is visible at a time. All other layers are turned off by modifying the output of Generate Webmap for Report and setting visibility=false for those layers. The basemap (either aerial imagery or a vector tiled topo map) is visible too, as is a graphics layer containing a handful of user-drawn graphics. 

Does anyone have an idea why it is so slow, or how to make reporting faster in general?