To be more detailed: It seems to be specific to how Samsung wires up the modem interfaces of Exynos devices. On Exynos4 devices, they're always hung off of HSIC (a variant of USB) buses. Since HSIC is based on USB, it has the same slow suspend/resume of USB. Almost all Exynos3 devices hung the modem off of a serial port if I recall correctly, and these devices could suspend/resume much faster. If you look at a resume sequence, there are huge time gaps, and after the first 200-300 msec, almost all resume activity is modem stuff.
The only solution, unless someone figures out a way to make the HSIC resume faster (I'm not messing with it, I have enough trouble getting it not to drop the bus randomly!!!!!!!), is to inhibit deep sleep with a wakelock - but this will be bad for battery life.