hi michelle,
this is in no way intended to serve as documentation or an official benchmark to compare and contrast the performance of the two APIs, but you might be interested in checking it out anyway.
Comparing Leaflet to ArcGIS JS API (renderers)
In general, Leaflet is a lighterweight (and therefore faster loading API) but the performance of the services which are called within the application and considerations in the code of the application itself are going to be a lot more noticeable to end users than a few milliseconds on page load.