Available with Standard or Advanced license.
The mosaic dataset item references a large non-tiled raster dataset.
When the mosaic dataset item references a large nontiled raster dataset, the display performance of the mosaic dataset may become slow, since this format is not optimized for fast access.
There is no solution for this warning, unless you want to convert your source data.
If you want to convert your data to a single dataset, it is recommended that you create a TIFF with compression (such as JPEG for lossy or LZ77 for lossless).
Learn about the raster storage settings
Optionally, you can use the Split Raster tool to break your raster dataset into smaller files.