The DriveSmart 61 (same hardware as the DriveTrack 71) is noticeably faster than any Garmin automotive model that I have seen. Going back a few years.... in 2010 I had a Nuvi 3790 which was Garmin's first glass screen multi-touch device. Nice unit, but definitely underpowered. Then I got a Nuvi 3550 in 2012 and that was much faster. I thought it was great, until I got a Dezl 760 in 2013 and it was faster still. The DriveSmart 61 blows it out of the water, in spite of the fact that the screen is 1024x600 and the Dezl is only 800x480. If you want to test your GPS, try this map. I doubt that anything older than a DriveSmart 61 will even work.
https://boydsmaps.com/lidar-in-the-pines-sd/This is my system of putting raster imagery in an .img file. It's a simple grid of 48ft ft x 48ft (15m x 15m) polygons with a road and POI overlay. It works fine on my DriveSmart 61. A bit slow to initially render a view, but it can keep up at highway driving speed and has never crashed (although it disappears if you zoom out farther than .2 miles). This same map just crashes my Dezl 760. Didn't bother trying it on any of the older devices.
But the DriveTrack 71 renders raster imagery pretty fast - a lot faster than my Montana 600 for example. Initially it has to render it tile by tile, then as you drive, each tile is updated as needed, so there are always some small areas in the process of rendering at highway speeds. The farther you zoom in, the faster the screen updates (since there are fewer tiles). With 1-meter/pixel imagery, initial map rendering is very slow at a zoom setting of .3 miles. At 300 ft zoom setting, rendering is very fast. Anyway... for Garmin, it's fast. But, admittedly not as fast as raster imagery on my iPhone 6s Plus.
Here's a screenshot from this morning, driving 50 mph with this map, performance was fine:
https://boydsmaps.com/new-jersey-2012-orthophotography/
I used a "custom map" (.kmz file) for this, instead of Birdseye (.jnx). Actually, rendering speed felt about the same as birdseye. My understanding is that .jnx files can contain 5 zoom levels, but I haven't tried that yet (.kmz maps can only have one zoom level). So that will be my next test - if they really support multiple zoom levels, that should speed up rendering considerably (at the expense of file size). The .jnx files I have made so far only have a singe 1-meter level (this would be level 17 if using a format like .mbtiles).
BTW, if you enable City Navigator at the same time as raster imagery, there is very little impact on performance. I really only notice the difference when making a 90 degree turn, which causes a big screen update.