Hi Ganimede,
I'm glad to know you have found an answer to your performance issue even if I have to say I'm kind of surprised about how those notes could have solved Bex queries performance issue, but I guess it might depend about what objects are involved on those Bex queries. Interesting anyway 
Just to clarifies about sybmgmtdb database: it is used by the ASE Job Scheduler (which is used by DBACockpit) and contains information about jobs, schedules, job logs informations, internal procedures.
Best regards,
Victoria.