what are the things that we need to consider for better application performance
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Agreed with Danny and this is a broader subject. However, please refer to the following URL's for a better understanding of performance.
Interfaces: https://docs.appian.com/suite/help/21.1/SAIL_Performance.html
Database: https://community.appian.com/w/the-appian-playbook/197/database-performance-best-practices
Appian recommendations: https://docs.appian.com/suite/help/21.1/appian-recommendations.html
Performance monitoring: https://docs.appian.com/suite/help/21.1/Logging.html#monitoring-performance-and-usage