There are so many factors that would make the delay.
1. Network. Usually will be intermittently.
2. Big data source. I think you mean SSAS. If the SSAS has a large amount of data.
3. The connection mode. Live connection or Import mode. If it's live connection, the network and performance of the Server could be the bottleneck. If it's Import mode, the size of the dataset and the complexity of the reports could be the bottleneck.
4. The way how the data model is designed.
How to optimize your project.
1. Add more power to the Server.
Could you please mark the proper answer as solution or share the solution if it's convenient for you? That will be a big help to the others.