While it seems less efficient, and can be, at a busy aiport like Singapore, having security done at each gate provides more capacity to do checks. A centralised security check would be a choke point. I've never seen a 500 metre long line at security in Singapore like I have in other airports at peak times.
It is less efficient (overall), even if the pax are screened faster at the gate (debatable - everybody is comparing with USA TSA, well let's see what happens if TSA are running the SIN gate screening).
The physical footprint of the screening areas, the equipment, the staff - replicated at all the gates. If a similar sized airport had the same amount of equipment and same number of staff operating from a small number of central locations, you'd never wait in line at all. Is it better to have 10 individual queues at a supermarket or one queue that leads to ten operators? It's the latter, because it smooths out the waiting time and processes everybody quicker. Or you could do it with 5 operators and make a saving on staff - which is what most airports do.
It's not the fact that security is at each gate that raises the capacity - it's the higher number of staff and equipment.
SIN is also exclusively an I-I port, this would be a terrible model for D-I where the secondary screen isn't required (but is for I-D), and it also wouldn't accommodate a country who required transit pax to go through passport control. It's also less effective for flights originating in SIN.
For me, I'm not a fan, going through any screening is a hassle even if there's no wait - having to remove electronics etc - discard liquids (that's a deal breaker in itself) - it's so much nicer walking straight from the lounge to the gate and walking straight on board. I see why SIN does it this way as it suits their market, but I would hate to see this extended elsewhere (and I doubt it would).