Easy Tech TutsHow to Use Query Diagnostics in Power BI (Find Slow Steps) Easily 2026
By Impran M N
When a Power BI report takes forever to refresh, the slowdown is usually hiding in one specific transformation step, not the whole query. This guide covers turning on Query Diagnostics, reading its output, and using it to find and fix the bottleneck. It's aimed at analysts dealing with sluggish refreshes who need to know exactly where time is being lost.
01Access Query Diagnostics
In Power Query Editor, open the Tools tab and start a diagnostics session before running or refreshing the query you want to analyze.
02Identify slow steps
Review the diagnostics output, which breaks down time spent at each transformation step, to spot where the majority of load time is going.
03Analyze performance bottlenecks
Look for steps involving heavy operations like merges, joins, or complex custom columns, since these are common sources of slow query performance.
04Automate diagnostics in your workflow
Run diagnostics periodically as your data model grows, since new steps or data volume increases can introduce bottlenecks that weren't there before.
Frequently asked questions
Where do I find Query Diagnostics in Power BI?
It's located under the Tools tab in Power Query Editor, where you can start recording a diagnostics session before refreshing your query.
What kind of information does Query Diagnostics show?
It shows a detailed breakdown of time spent on each applied step, along with details about the underlying data source calls being made.
What typically causes slow steps in Power Query?
Merges, joins across large tables, and complex custom column formulas are common culprits, along with pulling more data from the source than necessary.
Can Query Diagnostics fix slow queries automatically?
No, it only identifies where time is being spent — you'll still need to manually optimize the flagged steps, such as filtering data earlier or simplifying transformations.
Watch the full walkthrough
The same steps, demonstrated on screen from start to finish.
