How to find MySQL users causing high load?
The performance schema provides statistics aggregated by account (user + host), by user, and by host.

See for example table:

performance_schema.events_statements_summary_by_account_by_event_name


Reference manual
https://dev.mysql.com/doc/refman/5.7/en/statement-summary-tables.html

This table will show statement statistics (aka, queries) broken down per account ... anyone causing heavy traffic (high number of queries) or expensive computation (a few very expensive and long latencies queries) should be very visible in these statistics.

[Edit] Example of query:

select `USER`, `HOST`, SUM(COUNT_STAR) AS TOTAL
  from events_statements_summary_by_account_by_event_name
  group by `USER`, `HOST`
  order by TOTAL DESC;


will show how many queries each account do, ordered by count descending.