Doesn't feel slow and cumbersome to me. Our other vehicles are the Jeep with the 5.7l Hemi V8 and the Stage-2-tuned WRX, so we're comparing with plenty quick vehicles. But I find "normal" acceleration on a highway on-ramp or when wanting to pass is plenty adequate.
Ours is replacing a demised Mazda 3 GT with the 2.5l 4-banger, and I find this is comparable enough. We're probably not going to be autocrossing it (I asked, the wife said No), but for routine highway and city driving we have no complaints.
The youngun (who normally drives the WRX) had the same "concerns" when we were looking at the Jetta originally, but it hasn't stopped him from borrowing it as often as he can (usually when he's our of gas ... again). :-)
That said, if I was the kind of person who'd be interested in the R-line or similar sport model, I'd probably be wishing for a bit more beef under the hood too. ;-)