How to find the system impact of a query in postgres
Add pg_stat_statements in shared preload libraries 
Create extension pg_stat_statements. 
Then query on pg_stat_statements to get time related stats of the query. 

Query run by can be tracked from pg_stat_activity for the currently running statements. 

For blocks you can query  as below 
SELECT
    activity.pid,
    activity.usename,
    activity.query,
    blocking.pid AS blocking_id,
    blocking.query AS blocking_query
FROM pg_stat_activity AS activity
JOIN pg_stat_activity AS blocking ON blocking.pid = ANY(pg_blocking_pids(activity.pid));