How much data to keep is a question to which every customer will have different answer. Normally, customers are happy to keep last three years of data in Qlik Sense application, and this is good enough for them to do comparisons and see trends.

In case you haven't taken this into consideration, it's possible that Qlik application is storing data of several years thereby increasing the need of CPU and RAM on the server. If the customer is okay with last few years then the application can be modified, and this will definitely help in improving the performance.