Which Micro SD cards are the best for Libreelec and what max card size is supported?
If my old s805 mxq LE box run flawlessly with a S*msung 64GB microSD card (UHS-1 speed) with a normal SD card adapter, then your s905/s905x box should working at least 64GB cards without any problems.
The overall speed is excellent, much better than stock Android firmware with Kodi from eMMC, for example if you max out you download speed with streaming content it isnt slow down anything (i had always problems and freezes with torrent streaming on Android).