1. In Moodle 4.4 access report builder page /reportbuilder/index.php
2. Click on "New Report" and name it "Enrolment Count".
3. Set the "Report Source" to "Course Participants".
4. Configure the columns as follows:

   - Course • Course ID number - No aggregation

   - Course • Course short name - No aggregation

   - Course • Course full name - No aggregation

   - Enrolment method • Name - No aggregation

   - Tenant • Tenant name - No aggregation

   - User • Username - No aggregation

   - Enrolment method • Plugin - No aggregation

   - Course • Course short name with link - No aggregation

   - Course • Course full name with link - No aggregation

5. It takes ~10 seconds to save the changes and render data
6. After setting "User • Username" to "Count distinct" it takes more than 2 minutes to save the changed and render data. Same 2+ minutes are required to view the report
7. Setting "User • Username" to "Count" increases execution time from 10 seconds up to 20 which is kinda workable

Here is the tool_excimer flame graph:

We could debug and catch the actual SQL that is being executed, see MDL-83718-original-sql.txt attached. Running it directly in the database gives the same timing: ~60 seconds (2 mins devided by 2 as RB basically runs the SQL twice - to get the count and then get the actual records).

The simplified version of the SQL takes 40 seconds to complete:

SELECT

    rbalias3.idnumber AS c0_idnumber,

    rbalias3.shortname AS c1_shortname,

    rbalias3.id AS c1_courseid,

    rbalias3.fullname AS c2_fullname,

    rbalias3.id AS c2_courseid,

    rbalias8.enrol AS c3_enrol,

    rbalias8.name AS c3_name,

    rbalias8.courseid AS c3_courseid,

    rbalias8.roleid AS c3_roleid,

    rbalias8.customint1 AS c3_customint1,

    rbalias8.enrol AS c5_enrol,

    COUNT(DISTINCT rbalias15.username) AS c6_username,

    rbalias3.shortname AS c7_courseshortnamewithlink,

    rbalias3.id AS c7_id,

    rbalias3.fullname AS c8_coursefullnamewithlink,

    rbalias3.id AS c8_id

FROM

    mdl_course rbalias3

LEFT JOIN

    mdl_context rbalias4 ON rbalias4.contextlevel = 50 AND rbalias4.instanceid = rbalias3.id

LEFT JOIN

    mdl_enrol rbalias8 ON rbalias8.courseid = rbalias3.id

LEFT JOIN

    mdl_user_enrolments rbalias9 ON rbalias9.userid <> 1

    AND rbalias9.enrolid = rbalias8.id

LEFT JOIN

    mdl_user rbalias15 ON rbalias15.id = rbalias9.userid AND rbalias15.deleted = 0

WHERE

    rbalias3.id != 1

GROUP BY

    c0_idnumber, 

    c3_enrol, c3_name, c3_courseid, c3_roleid, c3_customint1, c5_enrol,

    c7_courseshortnamewithlink, c7_id

ORDER BY

    c3_enrol ASC;

Without the COUNT DISTINCT the SQL completes within seconds.

Moodle site details: Moodle 4.4.4, MySQL 8.0.36, PHP 8.1.2.1.2.19
Number of records in the involved tables:

mdl_course - 500

mdl_context - 40K

mdl_enrol - 4K

mdl_user_enrolments - 550K

mdl_user - 33K