If the Map service shows artifacts in your map display like jagged lines, wavy lines or bands, you can use Anti-aliasing for smoother display. Not all your map services would display artifacts. If it does, "Normal" is a good method to start with.
If the rendering is slow, you could then change it to "Fast". If the artifacts are still visible, you might change it to "Best".
Antialiasing with multiple cached resources | ArcGIS Blog
Antialiasing text is the process of blending text font edges so that characters appear less jagged. Map drawing performance is not affected by text antialiasing. If text antialiasing is not needed for improving map display clarity, use None, because it will result in smaller image sizes on disk.
Hope it helps!!!
Thank You Jayanta, it is very helpful,
but do you think it will affect the service performance, the speed to appear at the clint side ???
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