On Sawyer's website a video for the bucket setup claims the filter can purify 5 gallons in less than 20 minutes.

Does that sound about right?

EDIT:

Upon further research, the filtering time above is not for the viral filter, but rather the biological filter only. The time they quote for the viral filter is 4 liters in about 15 minutes.

Does this sound similar to the flow rate you were getting?

Thanks for any information.