Diagnosis
Yeah, this one's common. The solid red light on a PFM400H means the filter is either not seated correctly or the electronic monitor hasn't been reset for the new cartridge. The slow flow is just the new filter's media being compacted.
How to Fix It
First, turn the filter housing a quarter-turn to the left (counter-clockwise) to unlock it, but don't pull it off. Push it firmly back in and turn it to the right (clockwise) until it clicks and locks. That re-seats it. Then, press and hold the reset button on top of the monitor for about 3 seconds until the light turns blue or green.
If That Didn't Work
The slow flow might be from air trapped in the system. Run the cold water for a full 2-3 minutes to purge all the air. If the light is still red after that, the battery in the electronic monitor might be dead and need replacing (it takes a CR2 battery).
Heads Up
When you install a new filter, you have to run the water for 5 minutes to flush it before drinking, per PUR. If you skip that, you'll get carbon bits and a weird taste for a while.