Micro SD on a Nuvi 1490

Yes, if the maps are stored on the SD card, then the GPS unit will need to constantly access the card to read the maps.

But this isn't a problem.

Is there a reason that you are concerned about it?

I think I understand what you are saying.

If your storing map information on the SD card the Nuvi will need to access it in order for it to work.
If you remove the card whatever is on the card will be gone.

It's not like the Nuvi is putting it in temporary memory, it needs the card inserted.

Edit: Motorcycle Mama types faster than me, she covered it in her post.

I was concerned about processing speed and/or delays

I wonder if the read speed of the card will be slower. I'm thinking of a large freeway interchange while traveling at 60 mph. Will the Garmin keep up?
My map is on my card for the first time with 2013.20. I did not notice any difference yesterday, but I was not driving very fast either.

Read speed? Negligible.

You won't notice any difference.

I would think that the map data on the SD card is accessed as the unit builds the route, but after that, as you are driving, the route data itself is accessed from main storage. If the unit recalculates a new route it accesses the SD card again to build the new route in storage. Just a guess.

I have both the map and the large JCV file loaded on my micro SD card in my 1490LMT. It works just fine with no noticeable lag.

The "route" and the map data are two separate things.

The calculation of the route occurs in the device memory, but the maps remain on, and are accessed from, the SD card (if that's where they reside).

I've had the full US & Can maps on the SD card of my 1450 for a while now, driving many miles at highway speeds and in all traffic conditions with no performance problems.