Just did a few quick tests, FWIW. Started up my Montana 600 with no card. It has a copy of City Navigator Lower 48 and a few of my own maps in internal memory. Took 13 seconds to start. Repeated and it took 14 seconds.
Next I formatted a 16gb class 10 card FAT32, left it blank and put it in the Montana. Startup took 19 seconds. Repeated the test and it took 21 seconds.
Finally I formatted a 2gb card FAT32, left it blank and put it in the Montana. Startup took 14 seconds. Repeated and it took 15 seconds.
These times are pretty similar, and not a big deal, but the bigger card does seem to slow startup. Was not willing to reformat my 32gb card, which I am using in another device, but I bet it would be a bit slower.
Garmin used to have an FAQ about this on their support site. Evidently the device does some kind of integrity check at boot and it takes longer with a bigger card.
My point was really that placing a bunch of files on a card would slow down the boot process. I keep my Montana pretty lean now, with just the stuff mentioned above in internal memory. When I tried using about 14gb of Birdseye imagery on the 16gb card, it took forever to startup, I would guess about a minute.
You have to find the balance that suits your needs best. When the Montana was new, it tended to crash rather often. If I was using City Navigator in the car and the Montana crashed, I just wanted to get it restarted as quickly as possible.