it's plenty fast. i have a usb-attached nvme for the stuff i download to watch that I don't intend to keep, and my main collection on a separate NAS. the microsd I run coreelec from is a samsung evo, i think it's an A2 and it's plenty fast. I haven't installed coreelec to the emmc because i'm running nightly so it's a decent fallback when CE breaks which does happen once every few months - and allows me to easily make a backup using dd.

there was an issue with x265 playback being out of sync slightly (which could be remedied by seeking back or forwards one step, i used to just do it at the beginning of playback), which persisted for more than a year, but that was resolved about two weeks back, so aside from the occasional av1 issues, I'd consider the X4Q+ the best CE device i've ever tested. i had also used one of the homatics boxes that CE supports, but gave it to a friend.

One more note, I have this connected through an hdmi switch (that's shared with a steam deck and a ps5) to a usb-powered 4k hdr display. CEC works fine, passthrough audio works fine, color depth displays exactly, it's just a butter smooth experience.

i'm no ugoos evangelist, but the hardware is really solid and fairly cheap and hasn't failed me yet. i've been running this setup for about 2 years (iirc) and probably won't be needing to upgrade for a while.

edit: for clarification on speed and image caching, my library has roughly 1600 movies, 140 shows with about 5200 episodes, and everything is scraped. there's never any image loading issues like there is on slower hardware like the rpi and odroid.