When running Basecamp with the gps unit attached with the usb cable, there will be lag as the data has to be read through the cable. It's my belief that the etrex models run at the USB 1.0 specification speed and are a little sluggish naturally. I know of one way around this when working with Basecamp on your Mac computer.
Attach your gps unit to the computer with the USB cable and you should see the Garmin unit and the City Navigator SD card mount in the finder. My City Navigator mounts with the name 11-01318-52. Yours might have a different name as this probably depends on the map version you have. Start "Disk Utility" application which is included with all Macs. You will see the City Navigator SD card in the list of mounted disks in the sidebar of this program. Create a disk image of your SD card by going File>New>Disk Image from 11-01318-52 under the menus. This will create a file on your hard drive that has the same content as your SD card.
Now when you want to work in Basecamp with the maps, rather than use it with the gps system hooked to the computer, first go to "Disk Utility" and select the disk image you created in the sidebar then click on open. This will load the file in your memory of your computer as if the gps was attached. When you start Basecamp it will see the maps through the mounted disk image. However, it will be much faster as the data is in memory and not being read through a USB cable.
I hope I have explained that well enough, I am really not the best computer techy guy out there.