Macbook Pro Retina 13" performance for designers
If you can handle going from a 27" screen to a 13" screen (even though it's retina, the 'apparent' resolution is set quite low), then the rest of the system is fast.

Of course, you would want as much RAM and SSD as you can afford (you can't upgrade it later), but there is nothing to worry about with the CPU or the SSD speed.

In fact, the SSD alone will make it seem SUPER fast compared to a 3-4 year-old iMac (assuming it didn't have an SSD in it also).

If you needed an external GPU, then your only laptop Mac option these days is the top-end 15" Retina MBP, but that's larger and quite a bit more expensive than the 13" models.