What is the complexity of the leaflet function onEachFeature in general? is it the same as the complexity of a for loop when it comes to processing each key in a big geojson file?
onEachFeature is a user-defined function, and thus its algorithmical complexity cannot be known beforehand.
It does get called once per GeoJSON Feature on initialization of a L.GeoJSON instance, and whenever the .addData() method of a L.GeoJSON is called.
Also take into account that instantiating any L.Circle, L.Polyline or L.Polygon has a complexity of O(n*log(n)), where n is the number of points in the geometry, due to usage of a Douglas-Peucker simplification.
Thus, complexity of adding features to a L.GeoJSON should be of a magnitude like
O( m * (n*log(n) + f ) )
Where m is the number of features, n is the number of points/vertices per feature, and f is the complexity of the user-defined onEachFeature function.