Just like 2008, the 2IS doesn't have shifters.
It has a top gear limiter that you can control with either the stick or the paddles.
As such, if you use it thinking you are shifting the car, it will seem slow, because you're not shifting the car. The car is completely automatic. The only thing you can ever do is set a maximum top gear. Below that setting the car remains a fully automatic transmission.
The only change Lexus has made to this is from 2009+ you can use the paddles in D rather than needing to switch to S first (which I suppose could make things marginally quicker in certain situations, but what it's making quicker is still just locking out some upper gears, not actual shifting).
If you want virtually instant, real, shifting, without a clutch pedal, on an IS, you'll need an IS-F.
The 3rd gen IS (likely out next year as a 2013 model) will have significantly quicker shifting, though based on current news of the next-gen GS it may not be the "really manual" mode the IS-F has....which would be fairly disappointing.