Fast track at LGW (where EZY also operate) is only for passengers travelling in F or CW/Business. Which, as far as I'm aware, only applies to BA, EK, DL and QR.
No other people are supposed to use Fast Track, including BA silver or gold card holders travelling on Economy or WT+ tickets.

If EZY were to use it for, say, Speedy Boarding there'd be no point in it existing as the queues would be just as long as the regular security queues.

My understanding (and I could be wrong) is that the airlines have to pay the airport authority for providing it. If this is the case then the low cost nature of fares in Economy would probably prevent it, especially in the case of LCC's.