There is a known issue at beta 1 release on image service performance, the good news is that it has been improved and will be available at beta 2.
Please describe your data, the format, what raster type used when you create mosaic? any raster functions that you have applied? and what format did you use in GetImage request? If your data can be shared, please send your data to technical support. We can test it with the current software build.
Thank you for your testing.
Hong