After flashing a new ROM, the device can be slow while it rebuilds its cache. Give it a few hours. Consider installing an ultra-lightweight launcher (like "Lawnchair Launcher" (original version) or "Olauncher"). Avoid installing heavy apps like modern Facebook or YouTube; use their "Lite" versions or access them via a browser.
Many stock Arnova firmwares originally shipped with the "AppsLib" store instead of the official Google Play Store. Custom or modified ROMs add full Google Play Services. Arnova 10d G3 Firmware Rom
Ensure your tablet has at least 70% battery charge to prevent it from shutting down mid-flash. After flashing a new ROM, the device can