Improve sql server lookup table performance
I assume you mean a SQL Server instances hosted in an Azure VM (as opposed to a SQL Azure instance).


Use BINARY(16) for MD5 hash. Is half the size of CHAR(32)
Must have a clustered index on the hash
Choose an appropriate hosting plan. Read Premium Storage: High-Performance Storage for Azure Virtual Machine Workloads
Follow best practices. Read Performance Guidance for SQL Server in Azure Virtual Machines and Performance Best Practices for SQL Server in Azure Virtual Machine


Other than that, approach this as any other performance investigation. Measure. Read How to analyse SQL Server performance. The solution will depend a lot on what your measurements find.