On the big sites, with a lot of rows in mdl_role_capabilities table, this query is very slow:

    $sql = "SELECT ctx.path, rc.roleid, rc.capability, rc.permission

              FROM {role_capabilities} rc

              JOIN {context} ctx ON rc.contextid = ctx.id

             WHERE rc.roleid $sql

          ORDER BY ctx.path, rc.roleid, rc.capability";

See https://github.com/moodle/moodle/blob/master/lib/accesslib.php#L309 for the code of function get_role_definitions_uncached.

The slowness in MySQL and my data set comes from "ORDER BY ctx.path, rc.roleid, rc.capability". With ordering the query takes 10 minutes and without 25 seconds.
This is big data and run on slow and not-optimized machine, so the timings are a bit irrelevant - the speed up is.

In case of this query, ordering is not needed, the results go into associative array:

        $rdefs[$rd->roleid][$rd->path][$rd->capability] = (int) $rd->permission;