If you genuinely want the best of both worlds (size + speed), look for .CHD files that include the "Track 03.bin" audio. Some lazy converters drop the high-definition audio tracks. The "better" compressed files preserve the 44.1kHz stereo Redbook audio and compress it with FLAC. You can identify these by checking if the compressed file size is more than 300MB —if a Dreamcast game is smaller than 200MB, it has likely been gutted. Avoid it. Aim for the 40-60% compression sweet spot.
Use chdman (included with MAME or RetroArch). dreamcast+games+highly+compressed+better
Even compressed games rely on read speeds. Use a high-quality, Class 10 MicroSD card (like a SanDisk Ultra or Samsung EVO) if you are playing via an ODE or a mobile emulator. If you genuinely want the best of both