Thanks so much @Daniel and all others!
It's now runningin linux optimised mode! I found this discussions helpful:
After creating the maxima-optimised image, it was not running properly in due of SELinux until I have done following:
restorecon -Frv /yourpath-to-moodledata/stack/The error from SELinux logs /var/log/audit/audit.log was this, when accessing the Healtcheck https://your-moodle/question/type/stack/healthcheck.php:
type=AVC msg=audit(1514452742.849:278923): avc: denied { execute } for pid=27983 comm="maxima-optimise" path="/path-to-moodledata/stack/maxima-optimised" dev="dm-2" ino=100669758 scontext=system_u:system_r:httpd_sys_script_t:s0 tcontext=unconfined_u:object_r:httpd_sys_rw_content_t:s0 tclass=file
type=SYSCALL msg=audit(1514452742.849:278923): arch=c000003e syscall=9 per=40000 success=no exit=-13 a0=20100000 a1=8000 a2=7 a3=12 items=0 ppid=25708 pid=27983 auid=4294967295 uid=48 gid=48 euid=48 suid=48 fsuid=48 egid=48 sgid=48 fsgid=48 tty=(none) ses=4294967295 comm="maxima-optimise" exe="/path-to-moodledata/stack/maxima-optimised" subj=system_u:system_r:httpd_sys_script_t:s0 key=(null)
type=PROCTITLE msg=audit(1514452742.849:278923): proctitle="/path-to-moodledata/stack/maxima-optimised"