It's the number of decisions it has to make. London to say Edinburgh on the A-1 is easy as you only have half a dozen points, but tell it to check every unclassified road and there are too many possibilities for it to remember the results of them all and pick the shortest or quickest. Remember it's a machine and can't discount even a crazy route via Cardiff until it's started checking the numbers.
You'll find city centre to city centre is worst as it might have enough memory to get from central London to the edge of Edinburgh by every possible initial route, but will run out when it combines every possible way into the second city.
Set the unit for quickest route.
Tell it to avoid minor roads.
If you like the minor roads add manual waypoints and split the route into a larger number or shorter ones.
Andy
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