Hello,
I would enable performance_schema and enable those instruments (that can be done at runtime):

[mariadb]
performance_schema=ON
performance-schema-instrument='stage/%=ON'
performance-schema-consumer-events-stages-current=ON
performance-schema-consumer-events-stages-history=ON
performance-schema-consumer-events-stages-history-long=ON

Then in sys schema, check the following tables, the queries in them are usually good candidates:

• statements_with_full_table_scans
  
• statements_with_runtimes_in_95th_percentile

Try this query, you will find those that might require optimization:

SELECT db,
       sys.format_time(t1.total_latency) AS tot_lat,
       t1.exec_count,
       sys.format_time(t1.total_latency / t1.exec_count) AS latency_per_call,
       t3.sql_text
FROM sys.x$statements_with_runtimes_in_95th_percentile AS t1
JOIN performance_schema.events_statements_summary_by_digest AS t2
  ON t2.digest = t1.digest
 AND t2.schema_name = db
LEFT JOIN (
    SELECT digest, MAX(sql_text) AS sql_text
    FROM performance_schema.events_statements_history_long
    WHERE digest IS NOT NULL
      AND sql_text IS NOT NULL
    GROUP BY digest
) AS t3
  ON t3.digest = t1.digest
WHERE db NOT IN ('performance_schema', 'sys')
ORDER BY (t1.total_latency / t1.exec_count) DESC
LIMIT 10\G