Eheim Pro II (2028) problem

Looks normal for a 2028, maybe a little weak, but not much. Generally these are setup with spraybars to increase the pressure at the output. Is the pump making much noise, ie any grinding, humming or bumping?

One of the main points for filter is surface disruption you should be using the spray bar with that filter.

Inside the canister, are you using the media that came with it? If not, what's in your eheim? Is the tubing to and from the canister pretty straight, without dips, kinks or loops? It should be as short as possible. Once I had a pump with almost 3 meters of tubing on the return and it significantly reduced the flow because it was coiled under the tank.