I ran some tests this past weekend and I think it is a tube sizing issue.
Set at 10gpd it pumped 32oz in about 7-8 mins [~50gpd]
The other tests weren't as exact as the container was empty when I checked it, but I know this:
6 gpd pumped 64 oz in less than 30 mins
2.5 gpd pumped 64 oz in less...