w5500 ethernet chip mac address

Hello,

I am working with a Leonardo Ethernet board, but i think this is exactly the same thing for Arduino with Ethernet Shield.

Here is my code:

#include <Ethernet2.h>

byte mac = { 0x90,0xA2,0xDA,0x10,0x20,0x30 };
IPAddress ip(192, 168, 0, 55);
EthernetServer server(80);

Ethernet.begin(mac, ip);
server.begin();

The mac address was taken from a sample code. I changed randomly the last digits.
Everything works fine, but a Windows computer was connected to the network. This windows computer was very very slow, but when i disconnect my leonardo, everything is fine.

I am wondering something: Do you know if the cause is this random address mac i have choose that cause problem on windows network stack ?

There is a sticker on the leonardo with a mac address. Do you think i should use this address ? Why communication is working with my custom mac address ?

Thanks