Segments: graph appears quickly, export takes forever


Hello everybody,

this behaviour of Matomo is creating problems for me and I do not understand how it could happen:

I have a large database and I use segments, I changed all my segments with the “Segmented reports are pre-processed” option and after few tries invalidating reports and re-archiving it works fine (segmented data appears quickly in the dashboard, including graphs).
When I try to export data from a graph (eg. monthly traffic for 12 months in CSV) the server takes hours to respond with mysql taking up a lot of CPU for a long time.
It looks like it is re-calculating the report from the visit log, I don’t understand how this could be since what I am asking for is the same data points that appear on the graph which gets generated quickly.

Is there anything wrong with my setup or is this a bug?
Any help appreciated!
Thank you,