The number of fields on a report should be kept to a minimum
Impact area
Performance
Severity
Warning
Affected element
Report
Rule ID
SF-0140
Impact
Retrieving unnecessary data is always inefficient and slows down performance.
Remediation
Exclude unnecessary columns. This is especially effective if you’re using a summary or matrix report and have Show Details enabled.
Time to fix
60 min
References
See the Salesforce documentation resource: Improve Report Performance > Best Practices