If your Hose timer is getting low pressure, then this article should help.
First, you want to flip the screen filter around, so that the cone is going into the timer instead of into the hose faucet
After this, you want to make sure that your timer is connected properly and not cross-threaded.
Then you should test your timer to see if there was any improvement.
NOTE: It is normal to see a drop in water pressure/PSI/GPM when attaching a hose-end device