How much improvement can Oracle database tuning deliver?

It depends on the bottleneck: SQL-level tuning typically gains 5%-30%, index and execution-plan rewrites 20%-50%, and storage I/O optimization over 50%. ZhongLianDa pinpoints bottlenecks with AWR, ASH and ADDM diagnostics and works across SQL, parameters, indexing and I/O. We start with a free health check that states the expected gains, and implement only after your confirmation.