The wired Ethernet is 10/100, so it does not support 500 Mbps speed. Wireless Ethernet speeds are as environment-dependent as they are hardware-dependent -- interference and signal strength are the primary determinants of the operation speed.  The base wifi on this model is 150 Mbps, but the optional ones are 433 or 867 -- you must have one of those cards to be seeing 250.  You can try upgrading to an Intel Wireless 9560 card, but check to make sure your router supports its features.  
I have an Inspiron 5593.  I have internet speed of 500 Mb/sec but am only getting about 250 WiFi and the ethernet (wired) is ONLY 95mb/sec even with Cat 6 cable.  Very frustrated.  Does anyone know if changing/upgrading the WiFi card  (I have MAC E86F3893B691)  would help this??  I believe I have all the upgrades installed from Dell Assist.
Do I need new Wifi Card to boost speed to 500 Mb/sec? Inspiron 5593