You've not mentioned whether these are ArcGIS Online hosted feature layers, or published from your own servers. Optimization will differ for each.
In general, attempting to display millions of features is not advisable, nor a good experience for the user of maps/apps that display those layers. There are a variety of techniques that can be applied, such as scale-dependent display, creating indexes on attributes that are important for queries, and more...
IMHO ArcGIS Enterprise does not represent an "automatic upgrade" in performance, simply because you are reliant upon your own infrastructure for publishing those features. That said, there are "advanced" enterprise configurations that can scale appropriately, including cloud hosted publishing solutions.
Using ArcGIS Online it is possible to have millions of features performantly displayed in web maps if you are smart about how they are displayed. I have recently been working with approx. 170 millions features for display and everything is very performant, however we are careful about how features are displayed and when. Displaying 170 million features at once would not be a good end-user experience, so we make decisions about scale-wise display of data to provide for the best user experience and performance.
The "layers take too long to load" isn't an actionable complaint unless we can take a closer look what you are trying to add or view. You can contact me off-community at bszukalski@esri.com if you'd like me to have a closer look...
