Bit late to this discussion, but among the first things I do for Dreamcast on my Pi (with Batocera/Flycast) is make sure I set the region stuff. For some reason, if you don’t set the region correctly in the options, your games will run out of sync. So I always set it to NTSC, RGB, USA, and English. I usually use Soul Calibur as a guide as you can see the opening movie run out of sync before you do this. Of course, this depends on your roms being NTSC. There’s a couple of other things you can do too, like turning off certain effects.
What are you running it on, Pi or PC? What are you experiencing with your DC stuff, is it not running so well? If you’re still around, let us know.