I've got a small feature layer (79 features, ~1MB in size including attribute table, which has a sizable html snippet for a popup). In arcmap, the features display near instantaneously. I've checked geometry (clean), checked the projection, made sure it had no Z or M values. It has the default spatial indexing for a file geodatabase prior to publication. I've lightly generalized the polygon boundaries. The data is published to ArcGIS Online using the same process I've used to publish other layers, but it has a noticeable lag in being displayed in ArcGIS online compared to other feature layers (some larger) also hosted on AGOL.
Any thoughts or suggestions?