I'm on the other hand quite happy with it. It took me (just now) 1-2 minutes to 'refresh GPS' for the first time today inside my flat next to the window. After that I know it works quicker.
My only gripe with Vod Sat Nav (Telmap) is the map. In some places it shows a roundabout instead of a flyover. Other than that it's pretty good..if you download the route and stick to it. You may have to wait a bit if you miss a turn and it has to dl a route and the signal is rubbish.
So my recommendation would be to switch it on before you set off in the morning. Once the gps 'bit' has warms up you're good to go, hopefully.