To test the speed of your hard disk:
Open the Disk Utility application from the Activities overview.
Choose the hard disk from the Storage Devices list. Information about the disk and its status will appear under Drive.
Click Start Read-Only Benchmark to test how fast data can be read from the disk or Start Read/Write Benchmark to test how fast data can be read from and written to the disk. The latter option will take longer to complete.
When the test is finished, the results will appear on the graph.