Optimizing my script; taking too long; distance between many objects PostGIS

I have many many points and want to calculate the shortest distance from each point to the closest line of a table with many many lines. I think my underneath code is correct, I however never get an answer because pgadmin just keeps 'waiting for query execution to complete'. How can I optimize this underneath script?

SELECT 
    max(br.id), max(to_timestamp(br.time)), max(at.linknummer), 
    min(ST_Distance(st_transform(br.geom,3857), at.geometry))
FROM
    schema1.data1 AS br,
    schema2.data2 AS at 
GROUP BY
    br.id, br.time