I went to test drive the Nikon S1 last weekend (intend to buy actually). It was slim, and Nikon quality, but when I powered up, there was a delay (the time lag). I am not sure the time lag is common to all slim digicams or just Nikon S1.
there'll usually be some performance different compared to the larger models. however, the Ixus 30 that I had (not as slim as the S1) performed pretty well with good start-up time, AF speed and shutter lag.