Trying to understand your BigQuery slot usage in more detail can be achieved by querying the INFORMATION_SCHEMA.JOBS_BY_PROJECT dataset.

This dataset includes all of the related metadata attached to every job that is performed for the project. So not only can you find details around execution length and slot milliseconds consumed, but also details about which datasets are being queried and also what the destination table is.

All of this metadata can be very handy for reviewing where there might be performance issues with some of your queries.