How would you speed up a slow Looker Studio report?

First diagnose: reduce the page to a single chart. If it is still slow the problem is the source; if not, it is chart volume.

In-tool levers are fewer charts per page, longer data freshness caching, the Extract Data connector to snapshot slow sources, and fewer fields per chart.

The bigger wins are usually upstream: aggregate into a summary table in BigQuery, partition and cluster it, and connect to that rather than to raw events. Blends are frequently the slowest element on a page and are worth materialising upstream where possible.