If you enable caching of "Cloud v3, Easy Cloud" when writing GI cache files, the time required for this can be higher than other cache data. Unlike "Surface points" and "Atmo, Cloud v2" cache data, the Cloud v3 and Easy Cloud caches are normally calculated on the fly during the final pass, but when you cache them into a file it has to do this in the prepass instead.
If it's taking too long, try reducing "Cloud GI quality". Even the "Low" setting might be sufficient because the caching will eliminate flicker. "Cloud GI max ray depth" also affects how long the cache generation takes. You can also reduce the "Ray-marching quality" on the cloud layer's Quality tab; this can be different from your final pass.