A report may take a long time to process or may even fail if:
There are lots of objects (i.e. keywords, creatives, etc.) within the report. This can result in a large amount of data rows to be processed.
Calculated columns and/or custom columns are included in the report. The system will need to recalculate the values for each object. Including non-standard metrics in the custom column definition (ie. specific conversion types or other custom columns) can increase processing time.
Many filters on metric columns, dimension columns, custom columns. The system will need to recalculate the values for each filter, which can increase processing time.
Selecting a large date range and viewing data by day. This can result in a large amount of data rows to be processed.
There are many columns within the report. This can result in a large amount of data columns to be processed.
Having any combination of the above will cause this reporting issue, and is expected behavior.
If the report processing time is slow, cancel the running report and start a new one. When running large reports, you can take these steps to avoid report processing failure and reduce the run time: