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I don't have a 4 but on the 4S I can confirm it is very smooth. I'm quite pleased.
Startup is not problem for me, for me is problem in turns and when I change direction. But, TT is slow all time, speed of refresh is slow (about 1second) with all POI off. This problem was not on "non retina" version (1.4?).
There is another problem that TomTom have finally acknowledged with iOS5 and 1.8 ... basically don't lose your in-app purchases because at the moment you CANNOT restore them
I did a new install for my 4S and when i tried to login to restore my purchases you get 'incorrect username / password' error .... despite being able to login to TomTom.com with same.
Spoke to them and they agreed that it was a bug and 1.9 would fix it but i'm now left without HD Traffic until they sort it