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Wonder if a factory reset could help? I would've suggested a new SD card or formatting the card but internal storage only, I've no idea if that's possible. I have restored my 800's storage before after someone provided the stock filesystem for it online.
Maybe you could try using a smaller subset of OSM maps and see if it makes any difference - that would at least eliminate the factory maps as an issue.
You've just made me want to load a route on the 1040 and see how fast it is...
That uses built-in storage or an SD card?
Are you using OSM or factory maps with it?
What are the route calculation settings? (on-road, popularity, etc)