ok...sorry i misunderstood. I took your export, added it to my dev environment (clean setup from https://jsreport.net/learn/windows with 1 change to add the license to the config). I then added all of my templates from our test server and did the export of your stuff.
When I exported just your items it took almost 11 seconds. This makes me think that it is doing something disk related to find what it needs and the iteration of the items to get what it needs is causing the slowdown.
When I watch what is happening on my machine in Resource Monitor, i can see that it's processing files on disk and spikes CPU and disk, but doesn't do much at all with memory.
I changed the backend to be PostgreSQL for templates and all of the speed issues have been fixed on the export.