Few questions here:
Are you using live connection or extract?
What's the volume of data coming into tableau?
Many factors associated with it.
Are you using the Live connection or Extract? The Extract is faster than the Live connection as you have large data set. This article will help you to connect the data Set Up Data Sources
Are you using Joins or Data Blending? This article will help you if you want to choose Blend or Join; Blend Your Data .