i have the acer n35 with tomtom on and have asked the question ..why does it take anything from 10 mins to an hour for tomtom to find the sats

once it has found them its fantastic but shouldent it be instant? steve off here has a sony incar sat nav and that finds the sats as soon as you put it on, i no its not just mine as to of my friends have them and thers do the same..
any ideas ?
the system did come with destinator on it but was told thats not as good as tomtom, as it wont accept full postcodes
thanks
andy