Remember that prior to the Ladbroke Grove accident, the speed limit from pretty much the platform ends at Paddington was 80mph, rising to 100mph at Kensal Green. This was tailored to the HST acceleration graph, meaning a continuous notch 5 power application could be made from Royal Oak taking you right up to linespeed without break.
It is now 50mph to Kensal Green, which adds about 2 mins to the Paddington-Reading journey, additionally the running brake test is performed at 40mph rather than above 100mph which adds yet more time. Then allow for defensive / efficient driving techniques, and the extra Mk3 in the formations, ATP (and adherence to the 125mph limit!) and you can see why Padd-Reading in 18 mins can't possibly be achievable any more.