I own two old (almost ten and five years old) Garmin Nüvi devices. From some years I started to use more the car GPS system, that is a lot more faster!
I do not know if modern Garmin units are really more faster than my old 2595, but probably they are... Or if I have to switch to Tom Tom...
Lot of times I suffer slow calculation performances when I am driving on very hurbanized areas. I drive in Europe.