Based on the Sawyer website, you have the biological filter, not the one for viruses.
I have the older version of the Sawyer filter (same as yours) and the rate of flow is a lot better than that.
Quick few questions....have you back flushed it with the hose attachment for the faucet?
That could blow out something in the filter that could be blocking, but would be surprised if this was an issue.
(You really can generate a lot of pressure with the faucet adaptor).
Otherwise, the only other thing that I can think of is if you have air in the filter that is disrupting the flow.
Beyond that, I don't have any other ideas of what it could be...maybe the length of the tubing that you have above and below the filter to create pressure to push the water down and on the back end to create the siphoning effect.