The getSamples method in ArcGIS Server Image Service can be used for measuring performance.
Enter geometry.
{"x":-19837193.917600676,"y":-3812345.7636769526,"spatialReference":{"wkid":102100,"latestWkid":3857}}
(Optional) Enter Mosaic Rule.
{"mosaicMethod":"esriMosaicCenter","sortField":"","ascending":true,"mosaicOperation":"MT_FIRST","multidimensionalDefinition":[{"variableName":"ALL_SFC_SW_DIR","dimensionName":"StdZ","values":[0],"isSlice":true}]}
(Optional) Enter pixel size.
{"x":611.4962262813791,"y":611.49622628138,"spatialReference":{"wkid":102100,"latestWkid":3857}}
If REST service asks login credentials and fails to retrieve samples, it means that permission is not set properly (during 10.7 to 10.8 migration).
Make service public using ArcGIS Server Manager web interface.
Use web browser instead of wget to get sample points after logging in with siteadmin.