Slow lever return after shortening brake line and bleeding
There are three possible things that could be causing this:


Too little mineral oil remaining in the system, did you top up the brakes after shortening them?. This would create a vacuum in the system and would prevent the lever from returning fully. *--EDIT on reflection I think this is unlikely with a Shimano brake as after oil falls below a certain level air would enter the system and the lever wouldn't work.
A blockage caused by grease in the needle creating a slow vacuum.
A broken spring in the lever. There isn't enough negative pressure in the system to force the lever to return so this is aided by a spring in the lever which also creates suction at the piston. Did you check the brakes prior to shortening the hose? This is a known failure for Shimano brakes.


My trouble shooting would be:


Check mineral oil level
Check the needle for a blockage
Take it to a LBS for some professional help