What is the largest number of vector features in a feature layer that you have ever published on ArcGIS Online as a feature service (not tiled)? What is the geometry type (point,line polygon)? How did you get it to perform?
I have a feature service with a table in it that contains over 1 million records. A million is nothing for a typical database server to handle, but as a AGOL REST endpoint, a query for all rows returned only 2000 records by default. A query for count only timed out. Similarly, a query with a specific where clause just spins and never respond back.