First of all, thanks for your answer.
We're running PHP 5.4.36 on the machine but unfortunately, we can not upgrade to PHP 7 that easily because other applications use the PHP server and we're not 100% sure they would be PHP 7-compatible.
However, with the same PHP version, our biggest report needed about 2 or 3 minutes to be processed some months ago and it now needs 6 or 7 minutes.
We're investigating to see if something else might explain this loss.
Meanwhile, there's something you said that surprised me :
the CSV-export will always be faster as it pretty much generates the export as fast as the database delivers it.
I agree that CSV-export is faster, but in our case, it has never been able to generate the export as fast as the database delivers it. For our largest report (40 000 rows / 75 columns), we have an SQL query that needs less than 30 seconds to run on our database server... but the report itself has always needed at least 2 minutes to be generated by myDBR (this does not include the download time from the server to the client).
I had always thought it was due to the time needed by myDBR to populate the CSV file with the data but if you say it's nearly immediate, what are these 2 minutes (that have now become 6 or 7 minutes) used for ?