Hi Vivek,
There could be lot of things that could slow down the storyboard.
1. Make sure you have sized your BI Platform for SAP Lumira. Refer Sizing and Deploying SAP BI 4 and SAP Lumira
2. Check your BEx Queries if that is causing any performance problems. Eliminate bringing any unnecessary data and bring in data only that is required.
3. Execute the BEx query in Analyser to see how it is executing to pin issues related to your back end system.
Regards
Shankar