How can I get better performance when querying a delta table?
To get better performance when querying a delta table, there are many things you can do.  

First, ensure you are using the Photon engine.  The Photon engine is designed to handle high-concurrency and low-latency queries.  If you are using DBSQL, Photon is included free of charge and is turned on by default.

Second, you can use different indexing techniques when writing a Delta table for better read performance.  Z-ordering and bloom filters are two effective techniques when used correctly.

Third, ensure your SQL Warehouse or compute resource is appropriately sized.  If you are trying to query a very large dataset from a very small warehouse, you will likely get slow query results.