I have a similar experience with my Bike Friday All-day: its also very stable and allow very high speed, downhill and on flat roads too.

From my several folding bikes, its the most stable one: the Brompton 1" is the worst, the Birdy is pretty good but I never reached as high speed as with the All-day. My Moulton Speed is as good as the All-day but its not a folding bike!

What is also excellent with the All-day is that its very agile, it allow to take fast very sharp curves.