Hi fishfacs,
The /ui/administration/excel-reports and /excel-reports.shtm#finished pages have a table of previously run reports and information about them.
As far as logging, the excel reports module has pretty sparing debug information (and the error level logs would already appear in the Mango/logs/ma.log file). One information about queuing reports, starting and finishing. That would be debug level logging on the com.infiniteautomation.mango.excelreports that you would be adding to Mango/classes/log4j2.xml (copy to Mango/overrides/classes/log4j2.xml for persistence on upgrade).
I am not aware of how much logging there is to be had in the Apache POI library. That would be the org.apache.poi package, and I would expect either debug or trace level logging.
An entry like what I'm referring to in the log4j2.xml file looks like:
<AsyncLogger includeLocation="true" name="org.apache.poi" level="trace"/>
In my experience, having some really large named ranges is a way to slow reports down. Defining a range only requires a starting cell and a second cell to specify the direction to expand. Some people are tempted to select entire rows or columns when selecting the cells to create the named range, but this is not necessary.
Also so far as Java slowing down, if you schedule a whole bunch of reports to run at the same time, such that they compete for memory, they may run significantly slower than if they were run in sequence.
If your report is taking minutes to run, you can download a few thread dumps from the /ui/administration/system-status/threads while it is running and send them to support@infiniteautomation.com and I will attempt to resolve what is taking the time.