Based on the information you shared, I will try to make some assumptions.
First thing to do, check the network latency between the Tableau server and the Azure Synapse environment. That is the 1st thing I would think about especially with live connections that are constantly querying data.
Second point, even though you've scaled your DWUs, the problem might lie in how Azure Synapse handles multiple concurrent queries. With Tableau sending 15 requests simultaneously, the resource contention might be high.
So in this case, you may need to adjust the resource classes for users and queries to ensure that more memory and compute resources are allocated to each query.
As a workaround to improve user experience, consider using Tableau extracts instead of live connections for reports that don't require real-time data.