When using an iPod type device on most car players connected by USB, the interface is pretty consistent. Like most things they do, this is controlled by Apple. While what you see on the car display may be limited to the standard Artist, Album, and Track names, your iPod is still using other tags like disc and track numbers and AlbumArtist for sorting.
You mention using Bluetooth, which means you must be using an iPod Touch. While there are several apps that will provide a solution for playing FLAC files locally, none of them will provide a direct path when connected to your car radio by USB. But they will work connected by Bluetooth. What you see on the display will still be limited to the tags that the car radio manufacturer has chosen to show. And it is highly likely the SD card or USB key playback will look pretty much the same.
Regardless, having a tag like AlbumArtist or anything else in your music files should not affect the performance of the iPod, the car player, or any other device. If they can't use a specific tag it is simply ignored. Other than embedded artwork, text tags take up little to no space compared to music even in the lossiest of files. So there isn't a space savings benefit to really compare.
But back to the original intent of your posted thread. If there is truly a difference of 60x as your test would indicate, perhaps this should be reconsidered by @Florian sometime. Maybe with the machines of today with faster processing power, the reasons for this difference in operation from the first design long ago are no longer necessary.