The Dell 5577 has a 1920x1080 resolution screen.
As long as you mirror the laptop's display to the monitor then there should be no performance loss. That means the same video signal is being set to both the laptop screen and the monitor so they display the exact same thing. Which means you will be limiting the resolution to 1920x1080 at most.
If you want to play games at max resolution on the monitor then that will mean performance will be less. The higher the resolution, the more graphically demanding the game will be.