I have this login behavior as well. After logging in, the progress bar will slide about halfway on the screen with the background, and then switches over to the apple logo where the progress bar slides the rest of the way and boots into my desktop.I enabled it today.
So far performance wise, it is fine.
However there is an UGLY glitch when you boot up the computer.
Previously, with FV enabled, and you reboot, you get to the FV Login screen which would display your current wallpaper as its background, you log in, and a progress bar appears on the wallpapered screen until you reach the desktop. Looks great, as it should be.
Now with Mojave, currently when you reboot, you are greeted with the "Mojave Day" default picture which seems unable to be changed, then you log in, then the screen glitches and you get the old black screen with the white Apple logo & progress bar until you reach your desktop. BLEH!
It truly looks ugly and clunky. Surprised they released it like that.
I have used FV since the beginning and now with Mojave (even all beta versions). Never had any problems and I see no downside in speed (but that could be beacause I have used it for so many years that I don’t know what I’m missing *smile*).I've heard people having problems with it (across all versions of macOS). Just wondering if lately its been working fine. Does it cause any performance degradation? Thanks.