You can use Azure Monitor to view cache metrics to troubleshoot query performance.
The key metrics for troubleshooting the cache are Cache hit percentage and Cache used percentage.
Possible scenario: Your current working data set cannot fit into the cache which causes a low cache hit percentage due to physical reads. Consider scaling up your performance level and rerun your workload to populate the cache.
Reference:
https://docs.microsoft.com/da-dk/azure/synapse-analytics/sql-data-warehouse/sql-data-warehouse-how-to-monitor-cache