Hi Jing,
I had to use my wi-fi connection, as I didn't have any other option when I took the online exam. I think I was lucky not to experience any issues, but I've heard some candidates had problems due to a connection drop during the exam.
I believe the reason why the wired connection is recommended is that it is usually more stable than the wi-fi. Anyway, the minimum speed requirements are 3Mbps in download and 2Mbps in upload, so even 30Mbps should be okay.
Gabriella