Hello, The limit is 8GB for inspiron 5421 nothing you can do about that i'm afraid but if a performance boost is your goal you could replace the traditional HDD with an ssd that will only help in startup times though.
Hello!  I know my laptop is very old but I still can't buy another one. So I decided to upgrade it. I went to a local service center (not Dell service center) and upgraded my ram. The laptop comes with one 4gb ram stick installed, my plan was to get another 4gb ram stick. But they ran out of ddr3l 4gb rams, so I installed a 8gb ram stick in the available slot. Now my laptop has 12gb total ram. But when I checked the laptop's manual, it only supports 8gb ram maximum. What should I do?  I still haven't seen any error. Should I remove it? 
Inspiron 14r 5421 ram upgrade