You're mistaken.
ABS actually releases the braking force with micro impulsions when the sensor detects the wheel is slipping.
It performs that detection (and subsequent action) several hundreds or thousands time per second.
Therefore, when it comes to braking on slippery surfaces, ABS performs better than a panicking human being.
So yes, on slippery ground, it takes longer to come to a stop. It's the law of physics and as such applies to every possible braking system (apart from air brake, obviously).
Actually, a well implemented ABS makes such difficult braking safer (you don't risk losing control) and more efficient (you can't apply such fast and precise brake/release sequences with your hands).
Current ABS are indeed able to adapt its actions depending on the leaning angle of the bike, thanks to an inertial sensor also.