Long back-rev for a coaster brake to engage? I have a brand-new bike with Sram i3 installed, which is a 3-speed gear hub  with coaster brake. The coaster brake has a good feel of modulation, but in order for it to engage braking, I will need to freewheel (not pedalling) for a bit (single digit meters?), and then I can engage the braking with a little back kick. If I pause forward pedalling immediately without coasting a bit and try to brake, I noticed I will need to back press almost half-a-rev before the braking is in effect. 

Is this to be expected? Or if there's anything I need to do to tune the coaster brake for the situation to improve? I don't know if I can always manage to find some distance to prepare for the brake. 

There is a front rim brake available though.
I have used plenty of coaster brakes and you do not need to coast before you can brake.
Move your feet till one of them has enough leverage when you start braking, change direction of movement and the brake engages.

If you need more time your brake needs adjustments or even repairs.
Braking should be as instant as it is with any other type of brake.