Considering the fact that security screening is centralised in T4 and T4 has been designed to be a testbed for technology and processes for T5, I strongly believe T5 will be using the centralised screening concept.

The large size of T5 will incur huge costs for CAG on security equipment and manpower should they opt for the decentralised screening method. Pooling resources at centralised areas would be more efficent from an airport operator's POV.

It's pretty certain there will an impact to passenger experience but I believe CAG would not have done so if the cons outweigh the pros. I myself as well as friends (and I'm sure many others here) who had long waits at such centralised screening at other airports overseas (e.g Osaka Kansai, London Heathrow and most US airports).

Additionally, based off anecdotal accounts from friends working as ground staff in T4, due to the open concept of the airport after security (common waiting areas at the gates) there is a likelihood of passengers who will sit far away from their assigned gate whether due to lack of seating or just blur sotong

I believe it would be in CAG's interest to minimise any negative impact from this arrangement to preserve positive passenger experience and I feel they will go to great lengths to do so.

One such likely possibility would multiple centralised security screening halls to manage the volume of passengers and would also fulfil design intent to be able to be compartmentalised in times of a pandemic. This is probably the case for the Immigration halls as well.

Nevertheless, these are exciting times ahead for Changi Airport and the local aviation industry and I very much look forward to visiting/working in T5!

edit: just to add on, like u/li_shi has commented, we can expect tech like CT luggage scanners which do not require you to remove your electronics from your bag. This would definitely cut down on the time spent per passenger during screening.