Batch Statistical API is taking too long

I have been using batch statistical API for downloading the statistical data for multiple polygons. The issue comes when the number of polygons gets a little high e.g. 0.1-0.2 Million in my case, though the API status from start to done does not take too long but the downloading of statistics takes about 6-8 hours. Is there a specific reason for this slow response or is it because the number of polygon are high.
If the API is designed to handle multiple batch fields, the large number of polygons should not be a problem.