Query taking long time to execute

Below is my query, its taking long time to execute around 5 to 10mins for just 12 records Please correct me If something wrong in the query.

in the query, I am using one function, xxmec_getprevyeardata which returns average value. please see the function, below the select statement.

the query should fetch within 10 to 15 seconds for JUN-2019 period(parameter value which I am passing)

SELECT
    lookup_code,
    meaning period,
    period period_name,
    SUM(NVL(regular,0)) regular,
    SUM(NVL(incidental,0)) incidental,
    SUM(NVL(regular,0))+SUM(NVL(incidental,0)) total,
    ROUND(xxmec_getprevyeardata(period,'PYD')/(xxmec_getprevyeardata(period,'NO_PERIODS'))) average,
    jrnl_plan
FROM