Deepak,
Document Analyser gives you the list of unused fields, the chart or object which consumes more memory and helps in identifying which expression have complex expression.
Based on this information, you have to explore the dashboard and find out the expression and identify or replace with suitable lighter expressions.
Performance improvement can be done at many levels, UI,script level,server level.
Not just removing the unused fields will improve the performance.
Untill unless exploring the dashboard, cannot say where to improve the performance in your dashboard.
however, general information about performance tuning and best practices are attached with this.
Ensure yourself, and experiment it on your dashboard.
Regards,
Sivasankar
