Dear Heiko,
Sorry, I've overseen your question during my parential holidays.
To get a good peformance it is necessary to run Maxima in a thread pool. For each evaluation of a question maxima is called multiple times. If the server needs to start a new maxima process every time, this will sum up to a considerable request time.
You can get the maxima pool and its docu here:
https://github.com/maths/stack_util_maximapool
See also my answer to Ralf-Peter Balkes "Maximapool.conf Frage" for configuration tipps regarding the maximapool.
Best regards
Fred